diff --git a/core/includes/theme.inc b/core/includes/theme.inc
index bab2af5..e16c8d2 100644
--- a/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -10,6 +10,7 @@
 
 use Drupal\Core\Cache\CacheBackendInterface;
 use Drupal\Core\Template\Attribute;
+use Drupal\Core\Theme\Registry;
 use Drupal\Core\Utility\ThemeRegistry;
 
 /**
@@ -136,10 +137,8 @@ function drupal_theme_initialize() {
  *    the same information as the $theme object. It should be in
  *    'oldest first' order, meaning the top level of the chain will
  *    be first.
- * @param $registry_callback
- *   The callback to invoke to set the theme registry.
  */
-function _drupal_theme_initialize($theme, $base_theme = array(), $registry_callback = '_theme_load_registry') {
+function _drupal_theme_initialize($theme, $base_theme = array()) {
   global $theme_info, $base_theme_info, $theme_engine, $theme_path;
   $theme_info = $theme;
   $base_theme_info = $base_theme;
@@ -266,15 +265,9 @@ function _drupal_theme_initialize($theme, $base_theme = array(), $registry_callb
     }
   }
   // Load twig as secondary always available engine.
-  // @todo Make twig the default engine and remove this. This is required
-  // because (by design) the theme system doesn't allow modules to register more
-  // than one type of extension. We need a temporary backwards compatibility
-  // layer to allow us to perform core-wide .tpl.php to .twig conversion.
+  // @todo Replace this hack with generic $theme->engine loading (including base
+  //   themes).
   include_once DRUPAL_ROOT . '/core/themes/engines/twig/twig.engine';
-
-  if (isset($registry_callback)) {
-    _theme_registry_callback($registry_callback, array($theme, $base_theme, $theme_engine));
-  }
 }
 
 /**
@@ -300,90 +293,13 @@ function theme_get_registry($complete = TRUE) {
   if (!isset($drupal_static_fast)) {
     $drupal_static_fast['registry'] = &drupal_static('theme_get_registry');
   }
-  $theme_registry = &$drupal_static_fast['registry'];
-
-  // Initialize the theme, if this is called early in the bootstrap, or after
-  // static variables have been reset.
-  if (!is_array($theme_registry)) {
-    drupal_theme_initialize();
-    $theme_registry = array();
-  }
-
-  $key = (int) $complete;
-
-  if (!isset($theme_registry[$key])) {
-    list($callback, $arguments) = _theme_registry_callback();
-    if (!$complete) {
-      $arguments[] = FALSE;
-    }
-    $theme_registry[$key] = call_user_func_array($callback, $arguments);
-  }
-
-  return $theme_registry[$key];
-}
-
-/**
- * Sets the callback that will be used by theme_get_registry().
- *
- * @param $callback
- *   The name of the callback function.
- * @param $arguments
- *   The arguments to pass to the function.
- */
-function _theme_registry_callback($callback = NULL, array $arguments = array()) {
-  static $stored;
-  if (isset($callback)) {
-    $stored = array($callback, $arguments);
-  }
-  return $stored;
-}
+  $registry = &$drupal_static_fast['registry'];
 
-/**
- * Gets the theme_registry cache; if it doesn't exist, builds it.
- *
- * @param $theme
- *   The loaded $theme object as returned by list_themes().
- * @param $base_theme
- *   An array of loaded $theme objects representing the ancestor themes in
- *   oldest first order.
- * @param $theme_engine
- *   The name of the theme engine.
- * @param $complete
- *   Whether to load the complete theme registry or an instance of the
- *   Drupal\Core\Utility\ThemeRegistry class.
- *
- * @return
- *   The theme registry array, or an instance of the
- *   Drupal\Core\Utility\ThemeRegistry class.
- */
-function _theme_load_registry($theme, $base_theme = NULL, $theme_engine = NULL, $complete = TRUE) {
-  if ($complete) {
-    // Check the theme registry cache; if it exists, use it.
-    $cached = cache()->get("theme_registry:$theme->name");
-    if (isset($cached->data)) {
-      $registry = $cached->data;
-    }
-    else {
-      // If not, build one and cache it.
-      $registry = _theme_build_registry($theme, $base_theme, $theme_engine);
-      // Only persist this registry if all modules are loaded. This assures a
-      // complete set of theme hooks.
-      if (module_load_all(NULL)) {
-        _theme_save_registry($theme, $registry);
-      }
-    }
-    return $registry;
-  }
-  else {
-    return new ThemeRegistry('theme_registry:runtime:' . $theme->name, 'cache', array('theme_registry' => TRUE));
+  // @todo DIC this.
+  if (!isset($registry)) {
+    $registry = new Registry($GLOBALS['theme_info'], $GLOBALS['base_theme_info'], $GLOBALS['theme_engine']);
   }
-}
-
-/**
- * Writes the theme_registry cache into the database.
- */
-function _theme_save_registry($theme, $registry) {
-  cache()->set("theme_registry:$theme->name", $registry, CacheBackendInterface::CACHE_PERMANENT, array('theme_registry' => TRUE));
+  return $registry->get($complete);
 }
 
 /**
@@ -391,6 +307,8 @@ function _theme_save_registry($theme, $registry) {
  *
  * This function should be called when modules are added to the system, or when
  * a dynamic system needs to add more theme hooks.
+ *
+ * @todo Remove this.
  */
 function drupal_theme_rebuild() {
   drupal_static_reset('theme_get_registry');
@@ -398,290 +316,6 @@ function drupal_theme_rebuild() {
 }
 
 /**
- * Process a single implementation of hook_theme().
- *
- * @param $cache
- *   The theme registry that will eventually be cached; It is an associative
- *   array keyed by theme hooks, whose values are associative arrays describing
- *   the hook:
- *   - 'type': The passed-in $type.
- *   - 'theme path': The passed-in $path.
- *   - 'function': The name of the function generating output for this theme
- *     hook. Either defined explicitly in hook_theme() or, if neither
- *     'function' nor 'template' is defined, then the default theme function
- *     name is used. The default theme function name is the theme hook prefixed
- *     by either 'theme_' for modules or '$name_' for everything else. If
- *     'function' is defined, 'template' is not used.
- *   - 'template': The filename of the template generating output for this
- *     theme hook. The template is in the directory defined by the 'path' key of
- *     hook_theme() or defaults to "$path/templates".
- *   - 'variables': The variables for this theme hook as defined in
- *     hook_theme(). If there is more than one implementation and 'variables'
- *     is not specified in a later one, then the previous definition is kept.
- *   - 'render element': The renderable element for this theme hook as defined
- *     in hook_theme(). If there is more than one implementation and
- *     'render element' is not specified in a later one, then the previous
- *     definition is kept.
- *   - 'preprocess functions': See theme() for detailed documentation.
- *   - 'process functions': See theme() for detailed documentation.
- * @param $name
- *   The name of the module, theme engine, base theme engine, theme or base
- *   theme implementing hook_theme().
- * @param $type
- *   One of 'module', 'theme_engine', 'base_theme_engine', 'theme', or
- *   'base_theme'. Unlike regular hooks that can only be implemented by modules,
- *   each of these can implement hook_theme(). _theme_process_registry() is
- *   called in aforementioned order and new entries override older ones. For
- *   example, if a theme hook is both defined by a module and a theme, then the
- *   definition in the theme will be used.
- * @param $theme
- *   The loaded $theme object as returned from list_themes().
- * @param $path
- *   The directory where $name is. For example, modules/system or
- *   themes/bartik.
- *
- * @see theme()
- * @see _theme_process_registry()
- * @see hook_theme()
- * @see list_themes()
- */
-function _theme_process_registry(&$cache, $name, $type, $theme, $path) {
-  $result = array();
-
-  // Processor functions work in two distinct phases with the process
-  // functions always being executed after the preprocess functions.
-  $variable_process_phases = array(
-    'preprocess functions' => 'preprocess',
-    'process functions'    => 'process',
-  );
-
-  $hook_defaults = array(
-    'variables' => TRUE,
-    'render element' => TRUE,
-    'pattern' => TRUE,
-    'base hook' => TRUE,
-  );
-
-  // Invoke the hook_theme() implementation, process what is returned, and
-  // merge it into $cache.
-  $function = $name . '_theme';
-  if (function_exists($function)) {
-    $result = $function($cache, $type, $theme, $path);
-    foreach ($result as $hook => $info) {
-      // When a theme or engine overrides a module's theme function
-      // $result[$hook] will only contain key/value pairs for information being
-      // overridden.  Pull the rest of the information from what was defined by
-      // an earlier hook.
-
-      // Fill in the type and path of the module, theme, or engine that
-      // implements this theme function.
-      $result[$hook]['type'] = $type;
-      $result[$hook]['theme path'] = $path;
-
-      // If function and file are omitted, default to standard naming
-      // conventions.
-      if (!isset($info['template']) && !isset($info['function'])) {
-        $result[$hook]['function'] = ($type == 'module' ? 'theme_' : $name . '_') . $hook;
-      }
-
-      if (isset($cache[$hook]['includes'])) {
-        $result[$hook]['includes'] = $cache[$hook]['includes'];
-      }
-
-      // If the theme implementation defines a file, then also use the path
-      // that it defined. Otherwise use the default path. This allows
-      // system.module to declare theme functions on behalf of core .include
-      // files.
-      if (isset($info['file'])) {
-        $include_file = isset($info['path']) ? $info['path'] : $path;
-        $include_file .= '/' . $info['file'];
-        include_once DRUPAL_ROOT . '/' . $include_file;
-        $result[$hook]['includes'][] = $include_file;
-      }
-
-      // If the default keys are not set, use the default values registered
-      // by the module.
-      if (isset($cache[$hook])) {
-        $result[$hook] += array_intersect_key($cache[$hook], $hook_defaults);
-      }
-
-      // The following apply only to theming hooks implemented as templates.
-      if (isset($info['template'])) {
-        // Prepend the current theming path when none is set.
-        if (!isset($info['path'])) {
-          $result[$hook]['template'] = $path . '/templates/' . $info['template'];
-        }
-        if ($type == 'module') {
-          // Add two render engines for modules.
-          // @todo Remove and make twig the default engine.
-          $render_engines = array(
-            '.twig' => 'twig',
-            '.tpl.php' => 'phptemplate'
-          );
-
-          // Find the best engine for this template.
-          foreach ($render_engines as $extension => $engine) {
-            // Render the output using the template file.
-            $template_file = $result[$hook]['template'] . $extension;
-            if (isset($info['path'])) {
-              $template_file = $info['path'] . '/' . $template_file;
-            }
-            if (file_exists($template_file)) {
-              $result[$hook]['template_file'] = $template_file;
-              $result[$hook]['engine'] = $engine;
-              break;
-            }
-          }
-        }
-      }
-
-      // Allow variable processors for all theming hooks, whether the hook is
-      // implemented as a template or as a function.
-      foreach ($variable_process_phases as $phase_key => $phase) {
-        // Check for existing variable processors. Ensure arrayness.
-        if (!isset($info[$phase_key]) || !is_array($info[$phase_key])) {
-          $info[$phase_key] = array();
-          $prefixes = array();
-          if ($type == 'module') {
-            // Default variable processor prefix.
-            $prefixes[] = 'template';
-            // Add all modules so they can intervene with their own variable
-            // processors. This allows them to provide variable processors even
-            // if they are not the owner of the current hook.
-            $prefixes += module_list();
-          }
-          elseif ($type == 'theme_engine' || $type == 'base_theme_engine') {
-            // Theme engines get an extra set that come before the normally
-            // named variable processors.
-            $prefixes[] = $name . '_engine';
-            // The theme engine registers on behalf of the theme using the
-            // theme's name.
-            $prefixes[] = $theme;
-          }
-          else {
-            // This applies when the theme manually registers their own variable
-            // processors.
-            $prefixes[] = $name;
-          }
-          foreach ($prefixes as $prefix) {
-            // Only use non-hook-specific variable processors for theming hooks
-            // implemented as templates. See theme().
-            if (isset($info['template']) && function_exists($prefix . '_' . $phase)) {
-              $info[$phase_key][] = $prefix . '_' . $phase;
-            }
-            if (function_exists($prefix . '_' . $phase . '_' . $hook)) {
-              $info[$phase_key][] = $prefix . '_' . $phase . '_' . $hook;
-            }
-          }
-        }
-        // Check for the override flag and prevent the cached variable
-        // processors from being used. This allows themes or theme engines to
-        // remove variable processors set earlier in the registry build.
-        if (!empty($info['override ' . $phase_key])) {
-          // Flag not needed inside the registry.
-          unset($result[$hook]['override ' . $phase_key]);
-        }
-        elseif (isset($cache[$hook][$phase_key]) && is_array($cache[$hook][$phase_key])) {
-          $info[$phase_key] = array_merge($cache[$hook][$phase_key], $info[$phase_key]);
-        }
-        $result[$hook][$phase_key] = $info[$phase_key];
-      }
-    }
-
-    // Merge the newly created theme hooks into the existing cache.
-    $cache = $result + $cache;
-  }
-
-  // Let themes have variable processors even if they didn't register a
-  // template.
-  if ($type == 'theme' || $type == 'base_theme') {
-    foreach ($cache as $hook => $info) {
-      // Check only if not registered by the theme or engine.
-      if (empty($result[$hook])) {
-        foreach ($variable_process_phases as $phase_key => $phase) {
-          if (!isset($info[$phase_key])) {
-            $cache[$hook][$phase_key] = array();
-          }
-          // Only use non-hook-specific variable processors for theming hooks
-          // implemented as templates. See theme().
-          if (isset($info['template']) && function_exists($name . '_' . $phase)) {
-            $cache[$hook][$phase_key][] = $name . '_' . $phase;
-          }
-          if (function_exists($name . '_' . $phase . '_' . $hook)) {
-            $cache[$hook][$phase_key][] = $name . '_' . $phase . '_' . $hook;
-            $cache[$hook]['theme path'] = $path;
-          }
-          // Ensure uniqueness.
-          $cache[$hook][$phase_key] = array_unique($cache[$hook][$phase_key]);
-        }
-      }
-    }
-  }
-}
-
-/**
- * Builds the theme registry cache.
- *
- * @param $theme
- *   The loaded $theme object as returned by list_themes().
- * @param $base_theme
- *   An array of loaded $theme objects representing the ancestor themes in
- *   oldest first order.
- * @param $theme_engine
- *   The name of the theme engine.
- */
-function _theme_build_registry($theme, $base_theme, $theme_engine) {
-  $cache = array();
-  // First, process the theme hooks advertised by modules. This will
-  // serve as the basic registry. Since the list of enabled modules is the same
-  // regardless of the theme used, this is cached in its own entry to save
-  // building it for every theme.
-  if ($cached = cache()->get('theme_registry:build:modules')) {
-    $cache = $cached->data;
-  }
-  else {
-    foreach (module_implements('theme') as $module) {
-      _theme_process_registry($cache, $module, 'module', $module, drupal_get_path('module', $module));
-    }
-    // Only cache this registry if all modules are loaded.
-    if (module_load_all(NULL)) {
-      cache()->set("theme_registry:build:modules", $cache, CacheBackendInterface::CACHE_PERMANENT, array('theme_registry' => TRUE));
-    }
-  }
-
-  // Process each base theme.
-  foreach ($base_theme as $base) {
-    // If the base theme uses a theme engine, process its hooks.
-    $base_path = dirname($base->filename);
-    if ($theme_engine) {
-      _theme_process_registry($cache, $theme_engine, 'base_theme_engine', $base->name, $base_path);
-    }
-    _theme_process_registry($cache, $base->name, 'base_theme', $base->name, $base_path);
-  }
-
-  // And then the same thing, but for the theme.
-  if ($theme_engine) {
-    _theme_process_registry($cache, $theme_engine, 'theme_engine', $theme->name, dirname($theme->filename));
-  }
-
-  // Finally, hooks provided by the theme itself.
-  _theme_process_registry($cache, $theme->name, 'theme', $theme->name, dirname($theme->filename));
-
-  // Let modules alter the registry.
-  drupal_alter('theme_registry', $cache);
-
-  // Optimize the registry to not have empty arrays for functions.
-  foreach ($cache as $hook => $info) {
-    foreach (array('preprocess functions', 'process functions') as $phase) {
-      if (empty($info[$phase])) {
-        unset($cache[$hook][$phase]);
-      }
-    }
-  }
-  return $cache;
-}
-
-/**
  * Returns a list of all currently available themes.
  *
  * Retrieved from the database, if available and the site is not in maintenance
@@ -958,7 +592,7 @@ function drupal_find_base_themes($themes, $key, $used_keys = array()) {
 function theme($hook, $variables = array()) {
   // If called before all modules are loaded, we do not necessarily have a full
   // theme registry to work with, and therefore cannot process the theme
-  // request properly. See also _theme_load_registry().
+  // request properly.
   if (!module_load_all(NULL) && !defined('MAINTENANCE_MODE')) {
     throw new Exception(t('theme() may not be called until all modules are loaded.'));
   }
@@ -1058,22 +692,20 @@ function theme($hook, $variables = array()) {
         include_once DRUPAL_ROOT . '/' . $include_file;
       }
     }
-    if (isset($base_hook_info['preprocess functions']) || isset($base_hook_info['process functions'])) {
+    if (isset($base_hook_info['preprocess']) || isset($base_hook_info['process'])) {
       $variables['theme_hook_suggestion'] = $hook;
       $hook = $base_hook;
       $info = $base_hook_info;
     }
   }
-  if (isset($info['preprocess functions']) || isset($info['process functions'])) {
+  if (isset($info['preprocess']) || isset($info['process'])) {
     $variables['theme_hook_suggestions'] = array();
-    foreach (array('preprocess functions', 'process functions') as $phase) {
+    foreach (array('preprocess', 'process') as $phase) {
       if (!empty($info[$phase])) {
         foreach ($info[$phase] as $processor_function) {
-          if (function_exists($processor_function)) {
-            // We don't want a poorly behaved process function changing $hook.
-            $hook_clone = $hook;
-            $processor_function($variables, $hook_clone);
-          }
+          // We don't want a poorly behaved process function changing $hook.
+          $hook_clone = $hook;
+          $processor_function($variables, $hook_clone);
         }
       }
     }
@@ -1106,12 +738,10 @@ function theme($hook, $variables = array()) {
 
   // Generate the output using either a function or a template.
   $output = '';
-  if (isset($info['function'])) {
-    if (function_exists($info['function'])) {
-      $output = $info['function']($variables);
-    }
+  if (!isset($info['template']) && isset($info['function'])) {
+    $output = $info['function']($variables);
   }
-  else {
+  elseif (isset($info['template'])) {
     // Default render function and extension.
     $render_function = 'theme_render_template';
     $extension = '.tpl.php';
@@ -1135,32 +765,13 @@ function theme($hook, $variables = array()) {
       }
     }
 
-    // In some cases, a template implementation may not have had
-    // template_preprocess() run (for example, if the default implementation is
-    // a function, but a template overrides that default implementation). In
-    // these cases, a template should still be able to expect to have access to
-    // the variables provided by template_preprocess(), so we add them here if
-    // they don't already exist. We don't want to run template_preprocess()
-    // twice (it would be inefficient and mess up zebra striping), so we use the
-    // 'directory' variable to determine if it has already run, which while not
-    // completely intuitive, is reasonably safe, and allows us to save on the
-    // overhead of adding some new variable to track that.
-    if (!isset($variables['directory'])) {
-      $default_template_variables = array();
-      template_preprocess($default_template_variables, $hook);
-      $variables += $default_template_variables;
-    }
-
     // Render the output using the template file.
-    $template_file = $info['template'] . $extension;
-    if (isset($info['path'])) {
-      $template_file = $info['path'] . '/' . $template_file;
-    }
-
-    // Modules can override this.
     if (isset($info['template_file'])) {
       $template_file = $info['template_file'];
     }
+    else {
+      $template_file = $info['path'] . '/' . $info['template'] . $extension;
+    }
 
     $output = $render_function($template_file, $variables);
   }
@@ -3129,18 +2740,24 @@ function drupal_common_theme() {
     // From theme.inc.
     'html' => array(
       'render element' => 'page',
+      'preprocess' => array('template_preprocess_html'),
+      'process' => array('template_process_html'),
       'template' => 'html',
     ),
     'page' => array(
       'render element' => 'page',
+      'preprocess' => array('template_preprocess_page'),
+      'process' => array('template_process_page'),
       'template' => 'page',
     ),
     'region' => array(
       'render element' => 'elements',
+      'preprocess' => array('template_preprocess_region'),
       'template' => 'region',
     ),
     'datetime' => array(
       'variables' => array('timestamp' => NULL, 'text' => NULL, 'attributes' => array(), 'html' => FALSE),
+      'preprocess' => array('template_preprocess_datetime'),
       'template' => 'datetime',
     ),
     'status_messages' => array(
@@ -3190,6 +2807,7 @@ function drupal_common_theme() {
     ),
     'item_list' => array(
       'variables' => array('items' => array(), 'title' => '', 'type' => 'ul', 'attributes' => array()),
+      'preprocess' => array('template_preprocess_item_list'),
     ),
     'more_help_link' => array(
       'variables' => array('url' => NULL),
@@ -3212,6 +2830,8 @@ function drupal_common_theme() {
     // From theme.maintenance.inc.
     'maintenance_page' => array(
       'variables' => array('content' => NULL, 'show_messages' => TRUE),
+      'preprocess' => array('template_preprocess_maintenance_page'),
+      'process' => array('template_process_maintenance_page'),
       'template' => 'maintenance-page',
     ),
     'update_page' => array(
@@ -3254,6 +2874,7 @@ function drupal_common_theme() {
     ),
     'menu_tree' => array(
       'render element' => 'tree',
+      'preprocess' => array('template_preprocess_menu_tree'),
     ),
     'menu_local_task' => array(
       'render element' => 'element',
@@ -3267,6 +2888,7 @@ function drupal_common_theme() {
     // From form.inc.
     'input' => array(
       'render element' => 'element',
+      'preprocess' => array('template_preprocess_input'),
     ),
     'select' => array(
       'render element' => 'element',
diff --git a/core/includes/theme.maintenance.inc b/core/includes/theme.maintenance.inc
index ba88661..7baa164 100644
--- a/core/includes/theme.maintenance.inc
+++ b/core/includes/theme.maintenance.inc
@@ -90,9 +90,11 @@ function _drupal_maintenance_theme() {
 
 /**
  * Builds the registry when the site needs to bypass any database calls.
+ *
+ * @todo Remove this.
  */
 function _theme_load_offline_registry($theme, $base_theme = NULL, $theme_engine = NULL) {
-  return _theme_build_registry($theme, $base_theme, $theme_engine);
+  return theme_get_registry();
 }
 
 /**
diff --git a/core/lib/Drupal/Core/Theme/Registry.php b/core/lib/Drupal/Core/Theme/Registry.php
new file mode 100644
index 0000000..ac2232f
--- /dev/null
+++ b/core/lib/Drupal/Core/Theme/Registry.php
@@ -0,0 +1,641 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Core\Theme\Registry.
+ */
+
+namespace Drupal\Core\Theme;
+
+use Drupal\Component\Utility\NestedArray;
+use Drupal\Core\Cache\CacheBackendInterface;
+use Drupal\Core\Utility\ThemeRegistry;
+
+/**
+ * Defines the theme registry service.
+ *
+ * @todo Replace local $registry variables in methods with $this->registry.
+ */
+class Registry {
+
+  /**
+   * The theme object representing the active theme for this registry.
+   *
+   * @var object
+   */
+  protected $theme;
+
+  /**
+   * An array of base theme objects.
+   *
+   * @var array
+   */
+  protected $base_themes;
+
+  /**
+   * The name of the theme engine of $theme.
+   *
+   * @var string
+   */
+  protected $engine;
+
+  /**
+   * The complete theme registry.
+   *
+   * @var array
+   *   An associative array keyed by theme hook names, whose values are
+   *   associative arrays containing the aggregated hook definition:
+   *   - type: The type of the extension the original theme hook originates
+   *     from; e.g., 'module' for theme hook 'node' of Node module.
+   *   - name: The name of the extension the original theme hook originates
+   *     from; e.g., 'node' for theme hook 'node' of Node module.
+   *   - theme path: The path to the extension denoted by 'type' and 'name'.
+   *     @todo Remove 'theme path', it's useless.
+   *   - includes: (optional) An array of include files to load when the theme
+   *     hook is executed by theme().
+   *   - file: (optional) A filename to add to 'includes', either prefixed with
+   *     the value of 'path', or the path of the extension implementing
+   *     hook_theme().
+   *   In case of a theme base hook, one of the following:
+   *   - variables: An associative array whose keys are variable names and whose
+   *     values are default values of the variables to use for this theme hook.
+   *   - render element: A string denoting the name of the variable name, in
+   *     which the render element for this theme hook is provided.
+   *   In case of a theme template file:
+   *   - path: The path to the template file to use. Defaults to the
+   *     subdirectory 'templates' of the path of the extension implementing
+   *     hook_theme(); e.g., 'core/modules/node/templates' for Node module.
+   *   - template: The basename of the template file to use, without extension
+   *     (as the extension is specific to the theme engine). The template file
+   *     is in the directory defined by 'path'.
+   *   - template_file: A full path and file name to a template file to use.
+   *     Allows any extension to override the effective template file.
+   *   - engine: The theme engine to use for the template file.
+   *   In case of a theme function:
+   *   - function: The function name to call to generate the output.
+   *   For any registered theme hook, including theme hook suggestions:
+   *   - preprocess: An array of theme variable preprocess callbacks to invoke
+   *     before invoking final theme variable processors.
+   *   - process: An array of theme variable process callbacks to invoke
+   *     before invoking the actual theme function or template.
+   */
+  protected $registry;
+
+  /**
+   * The incomplete, runtime theme registry.
+   *
+   * @var \Drupal\Core\Utility\ThemeRegistry
+   */
+  protected $runtimeRegistry;
+
+  /**
+   * Constructs a new theme registry instance.
+   *
+   * @param object $theme
+   *   An object with (at least) the following information:
+   *   - uri: The path to the theme.
+   *   - owner: The name of the theme's base theme.
+   *   - engine: The name of the theme engine to use.
+   * @param array $base_themes
+   *   (optional) An array of objects that represent the base themes of $theme,
+   *   each having the same properties as $theme above, ordered by base theme
+   *   hierarchy; i.e., the first element is the root of all themes.
+   * @param string $theme_engine
+   *   The name of the theme engine.
+   *
+   * @todo Remove $theme_engine, duplicates ->engine.
+   * @todo Stack all themes into a single $themes in appropriate order.
+   */
+  public function __construct($theme, array $base_themes = array(), $theme_engine) {
+    // @todo Inject Cache.
+    // @todo Inject new ModuleHandler.
+    $this->theme = $theme;
+    $this->base_themes = $base_themes;
+    $this->engine = $theme_engine;
+  }
+
+  /**
+   * Returns the complete theme registry.
+   *
+   * @param bool $complete
+   *   Optional boolean to indicate whether to return the complete theme registry
+   *   array or an instance of the Drupal\Core\Utility\ThemeRegistry class.
+   *   If TRUE, the complete theme registry array will be returned. This is useful
+   *   if you want to foreach over the whole registry, use array_* functions or
+   *   inspect it in a debugger. If FALSE, an instance of the
+   *   Drupal\Core\Utility\ThemeRegistry class will be returned, this provides an
+   *   ArrayObject which allows it to be accessed with array syntax and isset(),
+   *   and should be more lightweight than the full registry. Defaults to TRUE.
+   *
+   * @return array|\Drupal\Core\Utility\ThemeRegistry
+   *   The complete theme registry array, or an instance of the
+   *   Drupal\Core\Utility\ThemeRegistry class.
+   *
+   * @todo Refactor calling code to remove $complete parameter.
+   */
+  public function get($complete = TRUE) {
+    if ($complete) {
+      if (!isset($this->registry)) {
+        $this->registry = $this->load();
+      }
+      return $this->registry;
+    }
+    else {
+      return $this->getRuntime();
+    }
+  }
+
+  /**
+   * Returns the incomplete, runtime theme registry.
+   *
+   * @return \Drupal\Core\Utility\ThemeRegistry
+   *   An shared instance of the ThemeRegistry class, provides an ArrayObject
+   *   that allows it to be accessed with array syntax and isset(), and is more
+   *   lightweight than the full registry.
+   */
+  public function getRuntime() {
+    if (!isset($this->runtimeRegistry)) {
+      $this->runtimeRegistry = new ThemeRegistry('theme_registry:runtime:' . $this->theme->name, 'cache', array('theme_registry' => TRUE));
+    }
+    return $this->runtimeRegistry;
+  }
+
+  /**
+   * Loads the theme registry from cache or rebuilds it.
+   *
+   * @param $complete
+   *   Whether to load the complete theme registry or an instance of the
+   *   Drupal\Core\Utility\ThemeRegistry class.
+   *
+   * @return array
+   *   The complete theme registry array.
+   *
+   * @todo Remove $complete argument. @see ::get()
+   */
+  public function load($complete = TRUE) {
+    if (!$complete) {
+      return $this->getRuntime();
+    }
+    // Check the theme registry cache; if it exists, use it.
+    $cached = cache()->get('theme_registry:' . $this->theme->name);
+    if (isset($cached->data)) {
+      $registry = $cached->data;
+    }
+    else {
+      // If not, build one and cache it.
+      $registry = $this->build();
+      // Only persist this registry if all modules are loaded. This assures a
+      // complete set of theme hooks.
+      if (module_load_all(NULL)) {
+        $this->save($registry);
+      }
+    }
+    return $registry;
+  }
+
+  /**
+   * Sets the theme registry cache.
+   *
+   * @param array $registry
+   *   The complete theme registry.
+   */
+  public function save($registry) {
+    cache()->set('theme_registry:' . $this->theme->name, $registry, CacheBackendInterface::CACHE_PERMANENT, array('theme_registry' => TRUE));
+  }
+
+  /**
+   * Builds the theme registry from scratch.
+   *
+   * Theme hook definitions are collected in the following order:
+   * - Modules
+   * - Base theme engines
+   * - Base themes
+   * - Theme engine
+   * - Theme
+   *
+   * All theme hook definitions are essentially just collated and merged in the
+   * above order. However, various extension-specific default values and
+   * customizations are required; e.g., to record the effective file path for
+   * theme template. Therefore, this method first collects all extensions per
+   * type, and then dispatches the processing for each extension to
+   * processExtension().
+   *
+   * @see hook_theme()
+   *
+   * After completing the collection, modules are allowed to alter it. Lastly,
+   * any derived and incomplete theme hook definitions that are hook suggestions
+   * for base hooks (e.g., 'block__node' for the base hook 'block') need to be
+   * determined based on the full registry and classified as 'base hook'.
+   *
+   * @see theme()
+   * @see hook_theme_registry_alter()
+   */
+  public function build() {
+    // @todo Replace local $registry variables in methods with $this->registry.
+    $registry = array();
+
+    // hook_theme() implementations of modules are always the same.
+    if ($cached = cache()->get('theme_registry:build:modules')) {
+      $registry = $cached->data;
+    }
+    else {
+      foreach (module_implements('theme') as $module) {
+        $this->processExtension($registry, 'module', $module, $module, drupal_get_path('module', $module));
+      }
+      // Only cache this registry if all modules are loaded.
+      // @todo Get rid of these checks.
+      if (module_load_all(NULL)) {
+        cache()->set("theme_registry:build:modules", $registry, CacheBackendInterface::CACHE_PERMANENT, array('theme_registry' => TRUE));
+      }
+    }
+
+    // Process each base theme.
+    foreach ($this->base_themes as $base) {
+      // If the base theme uses a theme engine, process its hooks.
+      $base_path = dirname($base->uri);
+      if ($this->engine) {
+        $this->processExtension($registry, 'base_theme_engine', $this->engine, $base->name, $base_path);
+      }
+      $this->processExtension($registry, 'base_theme', $base->name, $base->name, $base_path);
+    }
+
+    // And then the same thing, but for the theme.
+    if ($this->engine) {
+      $this->processExtension($registry, 'theme_engine', $this->engine, $this->theme->name, dirname($this->theme->uri));
+    }
+    // Finally, hooks provided by the theme itself.
+    $this->processExtension($registry, 'theme', $this->theme->name, $this->theme->name, dirname($this->theme->uri));
+
+    // Let modules alter the registry.
+    drupal_alter('theme_registry', $registry);
+
+    // Map base hook suggestions and clean up unnecessary hooks.
+    foreach ($registry as $hook => &$info) {
+      if (empty($info['exists'])) {
+        // Determine whether this is a hook suggestion for an existing hook.
+        $base_hook = $hook;
+        // Iteratively strip everything after the last '__' delimiter, until an
+        // implementation is found.
+        while ($pos = strrpos($base_hook, '__')) {
+          $base_hook = substr($base_hook, 0, $pos);
+          if (isset($registry[$base_hook])) {
+            break;
+          }
+        }
+        if ($pos !== FALSE && $base_hook !== $hook) {
+          $info['base hook'] = $base_hook;
+          unset($info['exists']);
+        }
+        else {
+          unset($registry[$hook]);
+        }
+      }
+    }
+    return $registry;
+  }
+
+  /**
+   * Process a single implementation of hook_theme().
+   *
+   * @param array $registry
+   *   The theme registry that will eventually be cached.
+   * @param string $type
+   *   One of 'module', 'base_theme_engine', 'theme_engine', 'base_theme', or
+   *   'theme'.
+   * @param string $name
+   *   The name of the extension implementing hook_theme().
+   * @param string $theme_name
+   *   The name of the extension for which theme hooks are currently processed.
+   *   This equals $name for all extension types, except when $type is a theme
+   *   engine, in which case $theme_name and $theme_path are pointing to the
+   *   respective [base] theme the theme engine is associated with.
+   * @param string $theme_path
+   *   The directory of $theme_name; e.g., 'core/modules/node' or
+   *   'themes/bartik'.
+   *
+   * @see theme()
+   * @see hook_theme()
+   */
+  public function processExtension(&$registry, $type, $name, $theme_name, $theme_path) {
+    $function = $name . '_theme';
+    foreach ($function($registry, $type, $theme_name, $theme_path) as $hook => $info) {
+      // Ensure this hook key exists; it will be set either way.
+      $registry += array($hook => array(
+        'type' => $type,
+        'name' => $name,
+      ));
+
+      if (isset($info['file'])) {
+        $include_path = isset($info['path']) ? $info['path'] : $theme_path;
+        $info['includes'][] = $include_path . '/' . $info['file'];
+        unset($info['file']);
+      }
+
+      // An actual theme hook must define either 'variables' or a 'render element'.
+      if (isset($info['variables']) || isset($info['render element'])) {
+        $info['exists'] = TRUE;
+
+        // The effective path_to_theme() during theme().
+        // @todo This variable is so horribly broken... remove it.
+        $info['theme path'] = $theme_path;
+
+        if (isset($info['template'])) {
+          // A template implementation always takes precedence over functions.
+          // A potentially existing function pointer is obsolete.
+          unset($registry[$hook]['function']);
+          // Default the template path to the 'templates' directory of the
+          // extension, unless overridden.
+          if (!isset($info['path'])) {
+            $info['path'] = $theme_path . '/templates';
+          }
+          // Find the preferred theme engine for this module template.
+          // @todo Remove and make Twig the default engine.
+          if ($type == 'module') {
+            $render_engines = array(
+              '.twig' => 'twig',
+              '.tpl.php' => 'phptemplate',
+            );
+            foreach ($render_engines as $extension => $engine) {
+              // Render the output using the template file.
+              $template_file = $info['path'] . '/' . $info['template'] . $extension;
+              if (file_exists($template_file)) {
+                $info['template_file'] = $template_file;
+                $info['engine'] = $engine;
+                break;
+              }
+            }
+          }
+        }
+        // Otherwise, the implementation must be a function. However, functions
+        // do not need to be specified manually; the array key of the hook is
+        // expected to be taken over as function, unless overridden.
+        elseif (!isset($info['function'])) {
+          if ($type == 'module') {
+            $info['function'] = 'theme_' . $hook;
+          }
+          else {
+            $info['function'] = $name . '_' . $hook;
+          }
+        }
+      }
+      // If no 'variables' or 'render element' was defined, then this hook
+      // definition extends an existing, or defines data for a hook suggestion.
+      // Data for hook suggestions requires a full registry in order to check
+      // for base hooks; therefore it happens after per-extension processing as
+      // last step in Registry::build().
+      else {
+        // Revert the above theme engine hack for Twig, if the actual theme
+        // engine returns a template.
+        // @todo What an insane hack is this? Simply support multiple theme engines.
+        if ($type == 'theme_engine' && isset($info['template'])) {
+          unset($registry[$hook]['template_file']);
+        }
+      }
+
+      // If a template should be used, inject the default template_preprocess().
+      // Also applies if the default implementation is a function, but a theme
+      // overrides it to a template.
+      if (isset($info['template']) && !isset($registry[$hook]['preprocess'])) {
+        $registry[$hook] += array('preprocess' => array());
+
+        // Prepend global hook_preprocess() of modules.
+        $registry[$hook]['preprocess'] = array_merge($this->getHookImplementations('preprocess'), $registry[$hook]['preprocess']);
+
+        // Prepend the base template_preprocess().
+        array_unshift($registry[$hook]['preprocess'], 'template_preprocess');
+
+        // Once more for hook_process(), but only add the key if any enabled
+        // modules implement it, since there is no base template_process().
+        if ($implementations = $this->getHookImplementations('process')) {
+          $registry[$hook] += array('process' => array());
+          $registry[$hook]['process'] = array_merge($implementations, $registry[$hook]['process']);
+        }
+      }
+
+      // Merge this extension's theme hook definition into the existing.
+      $registry[$hook] = NestedArray::mergeDeep($registry[$hook], $info);
+
+      // Allow themes and theme engines to remove all (pre)process functions.
+      // @see hook_theme()
+      if (!empty($info['no preprocess'])) {
+        unset($registry[$hook]['preprocess']);
+        unset($registry[$hook]['no preprocess']);
+      }
+    }
+  }
+
+  /**
+   * Retrieves module hook implementations for a given theme hook name.
+   *
+   * @param string $hook
+   *   The hook name to discover.
+   *
+   * @return array
+   *   An array of module hook implementations; i.e., the actual function names.
+   */
+  protected function getHookImplementations($hook) {
+    $implementations = module_implements($hook);
+    $add_suffix = function ($extension) use ($hook) {
+      return $extension . '_' . $hook;
+    };
+    return array_map($add_suffix, $implementations);
+  }
+
+  /**
+   * Invalidates theme registry caches and rebuilds the theme registry.
+   *
+   * @return array
+   *   The complete, rebuilt theme registry.
+   */
+  public function rebuild() {
+    $this->reset();
+    return $this->get();
+  }
+
+  /**
+   * Forces the system to rebuild the theme registry.
+   *
+   * This function should be called when modules are added to the system, or when
+   * a dynamic system needs to add more theme hooks.
+   */
+  public function reset() {
+    unset($this->runtimeRegistry);
+    unset($this->registry);
+    cache()->invalidateTags(array('theme_registry' => TRUE));
+  }
+
+  /**
+   * Allows themes and/or theme engines to discover overridden theme functions.
+   *
+   * @param $cache
+   *   The existing cache of theme hooks to test against.
+   * @param $prefixes
+   *   An array of prefixes to test, in reverse order of importance.
+   *
+   * @return $implementations
+   *   The functions found, suitable for returning from hook_theme().
+   *
+   * @todo Currently unused. Convert this.
+   */
+  public function findFunctions($cache, $prefixes) {
+    $implementations = array();
+    $functions = get_defined_functions();
+
+    foreach ($cache as $hook => $info) {
+      foreach ($prefixes as $prefix) {
+        // Find theme functions that implement possible "suggestion" variants of
+        // registered theme hooks and add those as new registered theme hooks.
+        // The 'pattern' key defines a common prefix that all suggestions must
+        // start with. The default is the name of the hook followed by '__'. An
+        // 'base hook' key is added to each entry made for a found suggestion,
+        // so that common functionality can be implemented for all suggestions of
+        // the same base hook. To keep things simple, deep hierarchy of
+        // suggestions is not supported: each suggestion's 'base hook' key
+        // refers to a base hook, not to another suggestion, and all suggestions
+        // are found using the base hook's pattern, not a pattern from an
+        // intermediary suggestion.
+        $pattern = isset($info['pattern']) ? $info['pattern'] : ($hook . '__');
+        if (!isset($info['base hook']) && !empty($pattern)) {
+          $matches = preg_grep('/^' . $prefix . '_' . $pattern . '/', $functions['user']);
+          if ($matches) {
+            foreach ($matches as $match) {
+              $new_hook = substr($match, strlen($prefix) + 1);
+              $arg_name = isset($info['variables']) ? 'variables' : 'render element';
+              $implementations[$new_hook] = array(
+                'function' => $match,
+                $arg_name => $info[$arg_name],
+                'base hook' => $hook,
+              );
+            }
+          }
+        }
+        // Find theme functions that implement registered theme hooks and include
+        // that in what is returned so that the registry knows that the theme has
+        // this implementation.
+        if (function_exists($prefix . '_' . $hook)) {
+          $implementations[$hook] = array(
+            'function' => $prefix . '_' . $hook,
+          );
+        }
+      }
+    }
+    return $implementations;
+  }
+
+  /**
+   * Allows themes and/or theme engines to easily discover overridden templates.
+   *
+   * @param $cache
+   *   The existing cache of theme hooks to test against.
+   * @param $extension
+   *   The extension that these templates will have.
+   * @param $path
+   *   The path to search.
+   *
+   * @todo Currently unused. Convert this.
+   */
+  public function findTemplates($cache, $extension, $path) {
+    $implementations = array();
+
+    // Collect paths to all sub-themes grouped by base themes. These will be
+    // used for filtering. This allows base themes to have sub-themes in its
+    // folder hierarchy without affecting the base themes template discovery.
+    $theme_paths = array();
+    foreach (list_themes() as $theme_info) {
+      if (!empty($theme_info->base_theme)) {
+        $theme_paths[$theme_info->base_theme][$theme_info->name] = dirname($theme_info->filename);
+      }
+    }
+    foreach ($theme_paths as $basetheme => $subthemes) {
+      foreach ($subthemes as $subtheme => $subtheme_path) {
+        if (isset($theme_paths[$subtheme])) {
+          $theme_paths[$basetheme] = array_merge($theme_paths[$basetheme], $theme_paths[$subtheme]);
+        }
+      }
+    }
+    global $theme;
+    $subtheme_paths = isset($theme_paths[$theme]) ? $theme_paths[$theme] : array();
+
+    // Escape the periods in the extension.
+    $regex = '/' . str_replace('.', '\.', $extension) . '$/';
+    // Get a listing of all template files in the path to search.
+    $files = file_scan_directory($path, $regex, array('key' => 'name'));
+
+    // Find templates that implement registered theme hooks and include that in
+    // what is returned so that the registry knows that the theme has this
+    // implementation.
+    foreach ($files as $template => $file) {
+      // Ignore sub-theme templates for the current theme.
+      if (strpos($file->uri, str_replace($subtheme_paths, '', $file->uri)) !== 0) {
+        continue;
+      }
+      // Chop off the remaining '.tpl' extension. $template already has the
+      // rightmost extension removed, but there might still be more, such as with
+      // .tpl.php, which still has .tpl in $template at this point.
+      if (($pos = strpos($template, '.tpl')) !== FALSE) {
+        $template = substr($template, 0, $pos);
+      }
+      // Transform - in filenames to _ to match function naming scheme
+      // for the purposes of searching.
+      $hook = strtr($template, '-', '_');
+      if (isset($cache[$hook])) {
+        $implementations[$hook] = array(
+          'template' => $template,
+          'path' => dirname($file->uri),
+        );
+      }
+
+      // Match templates based on the 'template' filename.
+      foreach ($cache as $hook => $info) {
+        if (isset($info['template'])) {
+          $template_candidates = array($info['template'], str_replace($info['theme path'] . '/templates/', '', $info['template']));
+          if (in_array($template, $template_candidates)) {
+            $implementations[$hook] = array(
+              'template' => $template,
+              'path' => dirname($file->uri),
+            );
+          }
+        }
+      }
+    }
+
+    // Find templates that implement possible "suggestion" variants of registered
+    // theme hooks and add those as new registered theme hooks. See
+    // drupal_find_theme_functions() for more information about suggestions and
+    // the use of 'pattern' and 'base hook'.
+    $patterns = array_keys($files);
+    foreach ($cache as $hook => $info) {
+      $pattern = isset($info['pattern']) ? $info['pattern'] : ($hook . '__');
+      if (!isset($info['base hook']) && !empty($pattern)) {
+        // Transform _ in pattern to - to match file naming scheme
+        // for the purposes of searching.
+        $pattern = strtr($pattern, '_', '-');
+
+        $matches = preg_grep('/^' . $pattern . '/', $patterns);
+        if ($matches) {
+          foreach ($matches as $match) {
+            $file = $match;
+            // Chop off the remaining extensions if there are any. $template
+            // already has the rightmost extension removed, but there might still
+            // be more, such as with .tpl.php, which still has .tpl in $template
+            // at this point.
+            if (($pos = strpos($match, '.')) !== FALSE) {
+              $file = substr($match, 0, $pos);
+            }
+            // Put the underscores back in for the hook name and register this
+            // pattern.
+            $arg_name = isset($info['variables']) ? 'variables' : 'render element';
+            $implementations[strtr($file, '-', '_')] = array(
+              'template' => $file,
+              'path' => dirname($files[$match]->uri),
+              $arg_name => $info[$arg_name],
+              'base hook' => $hook,
+            );
+          }
+        }
+      }
+    }
+    return $implementations;
+  }
+
+}
diff --git a/core/modules/aggregator/aggregator.module b/core/modules/aggregator/aggregator.module
index 35cdc19..1d262d3 100644
--- a/core/modules/aggregator/aggregator.module
+++ b/core/modules/aggregator/aggregator.module
@@ -54,6 +54,7 @@ function aggregator_theme() {
     'aggregator_wrapper' => array(
       'variables' => array('content' => NULL),
       'file' => 'aggregator.pages.inc',
+      'preprocess' => array('template_preprocess_aggregator_wrapper'),
       'template' => 'aggregator-wrapper',
     ),
     'aggregator_categorize_items' => array(
@@ -63,6 +64,7 @@ function aggregator_theme() {
     'aggregator_feed_source' => array(
       'variables' => array('feed' => NULL),
       'file' => 'aggregator.pages.inc',
+      'preprocess' => array('template_preprocess_aggregator_feed_source'),
       'template' => 'aggregator-feed-source',
     ),
     'aggregator_block_item' => array(
@@ -71,15 +73,18 @@ function aggregator_theme() {
     'aggregator_summary_items' => array(
       'variables' => array('summary_items' => NULL, 'source' => NULL),
       'file' => 'aggregator.pages.inc',
+      'preprocess' => array('template_preprocess_aggregator_summary_items'),
       'template' => 'aggregator-summary-items',
     ),
     'aggregator_summary_item' => array(
       'variables' => array('item' => NULL),
+      'preprocess' => array('template_preprocess_aggregator_item'),
       'file' => 'aggregator.pages.inc',
     ),
     'aggregator_item' => array(
       'variables' => array('item' => NULL),
       'file' => 'aggregator.pages.inc',
+      'preprocess' => array('template_preprocess_aggregator_item'),
       'template' => 'aggregator-item',
     ),
     'aggregator_page_opml' => array(
@@ -90,6 +95,9 @@ function aggregator_theme() {
       'variables' => array('feeds' => NULL, 'category' => NULL),
       'file' => 'aggregator.pages.inc',
     ),
+    'block' => array(
+      'preprocess' => array('aggregator_preprocess_block'),
+    ),
   );
 }
 
diff --git a/core/modules/block/block.module b/core/modules/block/block.module
index d9155c3..c75e0c0 100644
--- a/core/modules/block/block.module
+++ b/core/modules/block/block.module
@@ -85,11 +85,13 @@ function block_theme() {
   return array(
     'block' => array(
       'render element' => 'elements',
-      'template' => 'block',
+      'preprocess' => array('template_preprocess_block'),
+     'template' => 'block',
     ),
     'block_admin_display_form' => array(
       'template' => 'block-admin-display-form',
       'file' => 'block.admin.inc',
+      'preprocess' => array('template_preprocess_block_admin_display_form'),
       'render element' => 'form',
     ),
   );
diff --git a/core/modules/book/book.module b/core/modules/book/book.module
index 99edfc5..366d64d 100644
--- a/core/modules/book/book.module
+++ b/core/modules/book/book.module
@@ -43,10 +43,12 @@ function book_theme() {
   return array(
     'book_navigation' => array(
       'variables' => array('book_link' => NULL),
+      'preprocess' => array('template_preprocess_book_navigation'),
       'template' => 'book-navigation',
     ),
     'book_export_html' => array(
       'variables' => array('title' => NULL, 'contents' => NULL, 'depth' => NULL),
+      'preprocess' => array('template_preprocess_book_export_html'),
       'template' => 'book-export-html',
     ),
     'book_admin_table' => array(
@@ -54,10 +56,12 @@ function book_theme() {
     ),
     'book_all_books_block' => array(
       'render element' => 'book_menus',
+      'preprocess' => array('template_preprocess_book_all_books_block'),
       'template' => 'book-all-books-block',
     ),
     'book_node_export_html' => array(
       'variables' => array('node' => NULL, 'children' => NULL),
+      'preprocess' => array('template_preprocess_book_node_export_html'),
       'template' => 'book-node-export-html',
     ),
   );
diff --git a/core/modules/comment/comment.module b/core/modules/comment/comment.module
index 7ebef3e..f2bc837 100644
--- a/core/modules/comment/comment.module
+++ b/core/modules/comment/comment.module
@@ -186,6 +186,9 @@ function comment_field_extra_fields() {
  */
 function comment_theme() {
   return array(
+    'block__comment' => array(
+      'preprocess' => array('comment_preprocess_block'),
+    ),
     'comment_block' => array(
       'variables' => array('number' => NULL),
     ),
@@ -193,6 +196,7 @@ function comment_theme() {
       'variables' => array('comment' => NULL),
     ),
     'comment' => array(
+      'preprocess' => array('template_preprocess_comment'),
       'template' => 'comment',
       'render element' => 'elements',
     ),
@@ -200,6 +204,7 @@ function comment_theme() {
       'variables' => array('node' => NULL),
     ),
     'comment_wrapper' => array(
+      'preprocess' => array('template_preprocess_comment_wrapper'),
       'template' => 'comment-wrapper',
       'render element' => 'content',
     ),
diff --git a/core/modules/field/field.module b/core/modules/field/field.module
index df37a5b..6f2ce2e 100644
--- a/core/modules/field/field.module
+++ b/core/modules/field/field.module
@@ -308,6 +308,8 @@ function field_theme() {
   return array(
     'field' => array(
       'render element' => 'element',
+      'preprocess' => array('template_preprocess_field'),
+      'process' => array('template_process_field'),
     ),
     'field_multiple_value_form' => array(
       'render element' => 'element',
diff --git a/core/modules/forum/forum.module b/core/modules/forum/forum.module
index fbe06a0..a65e6e0 100644
--- a/core/modules/forum/forum.module
+++ b/core/modules/forum/forum.module
@@ -61,23 +61,31 @@ function forum_help($path, $arg) {
  */
 function forum_theme() {
   return array(
+    'block__forum' => array(
+      'preprocess' => array('forum_preprocess_block'),
+    ),
     'forums' => array(
+      'preprocess' => array('template_preprocess_forums'),
       'template' => 'forums',
       'variables' => array('forums' => NULL, 'topics' => NULL, 'parents' => NULL, 'tid' => NULL, 'sortby' => NULL, 'forum_per_page' => NULL),
     ),
     'forum_list' => array(
+      'preprocess' => array('template_preprocess_forum_list'),
       'template' => 'forum-list',
       'variables' => array('forums' => NULL, 'parents' => NULL, 'tid' => NULL),
     ),
     'forum_topic_list' => array(
+      'preprocess' => array('template_preprocess_forum_topic_list'),
       'template' => 'forum-topic-list',
       'variables' => array('tid' => NULL, 'topics' => NULL, 'sortby' => NULL, 'forum_per_page' => NULL),
     ),
     'forum_icon' => array(
+      'preprocess' => array('template_preprocess_forum_icon'),
       'template' => 'forum-icon',
       'variables' => array('new_posts' => NULL, 'num_posts' => 0, 'comment_mode' => 0, 'sticky' => 0, 'first_new' => FALSE),
     ),
     'forum_submitted' => array(
+      'preprocess' => array('template_preprocess_forum_submitted'),
       'template' => 'forum-submitted',
       'variables' => array('topic' => NULL),
     ),
diff --git a/core/modules/help/help.module b/core/modules/help/help.module
index aac8636..6a51bb5 100644
--- a/core/modules/help/help.module
+++ b/core/modules/help/help.module
@@ -68,6 +68,16 @@ function help_help($path, $arg) {
 }
 
 /**
+ * Implements hook_theme().
+ */
+function help_theme() {
+  $theme['block__system_help_block'] = array(
+    'preprocess' => array('help_preprocess_block'),
+  );
+  return $theme;
+}
+
+/**
  * Implements hook_preprocess_HOOK() for block.tpl.php.
  */
 function help_preprocess_block(&$variables) {
diff --git a/core/modules/language/language.module b/core/modules/language/language.module
index c2356be..6acb7a1 100644
--- a/core/modules/language/language.module
+++ b/core/modules/language/language.module
@@ -188,6 +188,9 @@ function language_permission() {
  */
 function language_theme() {
   return array(
+    'block__language' => array(
+      'preprocess' => array('language_preprocess_block'),
+    ),
     'language_negotiation_configure_form' => array(
       'render element' => 'form',
     ),
diff --git a/core/modules/locale/locale.module b/core/modules/locale/locale.module
index 5ba05d2..1187ed8 100644
--- a/core/modules/locale/locale.module
+++ b/core/modules/locale/locale.module
@@ -259,6 +259,9 @@ function locale_permission() {
  */
 function locale_theme() {
   return array(
+    'node' => array(
+      'preprocess' => array('locale_preprocess_node'),
+    ),
     'locale_translate_edit_form_strings' => array(
       'render element' => 'form',
       'file' => 'locale.pages.inc',
diff --git a/core/modules/menu/menu.module b/core/modules/menu/menu.module
index 3d4a360..5317d8b 100644
--- a/core/modules/menu/menu.module
+++ b/core/modules/menu/menu.module
@@ -188,6 +188,9 @@ function menu_uri(Menu $menu) {
  */
 function menu_theme() {
   return array(
+    'block__menu' => array(
+      'preprocess' => array('menu_preprocess_block'),
+    ),
     'menu_overview_form' => array(
       'file' => 'menu.admin.inc',
       'render element' => 'form',
diff --git a/core/modules/node/node.module b/core/modules/node/node.module
index 9e8fbd3..53ee795 100644
--- a/core/modules/node/node.module
+++ b/core/modules/node/node.module
@@ -147,8 +147,12 @@ function node_help($path, $arg) {
  */
 function node_theme() {
   return array(
+    'block__node' => array(
+      'preprocess' => array('node_preprocess_block'),
+    ),
     'node' => array(
       'render element' => 'elements',
+      'preprocess' => array('template_preprocess_node'),
       'template' => 'node',
     ),
     'node_search_admin' => array(
diff --git a/core/modules/overlay/overlay.module b/core/modules/overlay/overlay.module
index 54eaf18..1ee332e 100644
--- a/core/modules/overlay/overlay.module
+++ b/core/modules/overlay/overlay.module
@@ -71,13 +71,24 @@ function overlay_permission() {
  */
 function overlay_theme() {
   return array(
+    'html' => array(
+      'preprocess' => array('overlay_preprocess_html'),
+    ),
+    'maintenance_page' => array(
+      'preprocess' => array('overlay_preprocess_maintenance_page'),
+    ),
     'overlay' => array(
       'render element' => 'page',
+      'preprocess' => array('template_preprocess_overlay'),
+      'process' => array('template_process_overlay'),
       'template' => 'overlay',
     ),
     'overlay_disable_message' => array(
       'render element' => 'element',
     ),
+    'page' => array(
+      'preprocess' => array('overlay_preprocess_page'),
+    ),
   );
 }
 
diff --git a/core/modules/poll/poll.module b/core/modules/poll/poll.module
index 85d29b4..9d3e593 100644
--- a/core/modules/poll/poll.module
+++ b/core/modules/poll/poll.module
@@ -34,7 +34,11 @@ function poll_help($path, $arg) {
  */
 function poll_theme() {
   $theme_hooks = array(
+    'block__poll' => array(
+      'preprocess' => array('poll_preprocess_block'),
+    ),
     'poll_vote' => array(
+      'preprocess' => array('template_preprocess_poll_vote'),
       'template' => 'poll-vote',
       'render element' => 'form',
     ),
@@ -42,6 +46,7 @@ function poll_theme() {
       'render element' => 'form',
     ),
     'poll_results' => array(
+      'preprocess' => array('template_preprocess_poll_results'),
       'template' => 'poll-results',
       'variables' => array('raw_title' => NULL, 'results' => NULL, 'votes' => NULL, 'raw_links' => NULL, 'block' => NULL, 'nid' => NULL, 'vote' => NULL),
     ),
diff --git a/core/modules/rdf/rdf.module b/core/modules/rdf/rdf.module
index 55d89cb..899a20c 100644
--- a/core/modules/rdf/rdf.module
+++ b/core/modules/rdf/rdf.module
@@ -437,6 +437,30 @@ function rdf_theme() {
     'rdf_metadata' => array(
       'variables' => array('metadata' => array()),
     ),
+    'comment' => array(
+      'preprocess' => array('rdf_preprocess_comment'),
+    ),
+    'field' => array(
+      'preprocess' => array('rdf_preprocess_field'),
+    ),
+    'html' => array(
+      'preprocess' => array('rdf_preprocess_html'),
+    ),
+    'image' => array(
+      'preprocess' => array('rdf_preprocess_image'),
+    ),
+    'node' => array(
+      'preprocess' => array('rdf_preprocess_node'),
+    ),
+    'taxonomy_term' => array(
+      'preprocess' => array('rdf_preprocess_taxonomy_term'),
+    ),
+    'user' => array(
+      'preprocess' => array('rdf_preprocess_user'),
+    ),
+    'username' => array(
+      'preprocess' => array('rdf_preprocess_username'),
+    ),
   );
 }
 
diff --git a/core/modules/search/search.module b/core/modules/search/search.module
index 941621c..3eb5c9a 100644
--- a/core/modules/search/search.module
+++ b/core/modules/search/search.module
@@ -103,14 +103,19 @@ function search_help($path, $arg) {
  */
 function search_theme() {
   return array(
+    'block__search_form_block' => array(
+      'preprocess' => array('search_preprocess_block'),
+    ),
     'search_result' => array(
       'variables' => array('result' => NULL, 'module' => NULL),
       'file' => 'search.pages.inc',
+      'preprocess' => array('template_preprocess_search_result'),
       'template' => 'search-result',
     ),
     'search_results' => array(
       'variables' => array('results' => NULL, 'module' => NULL),
       'file' => 'search.pages.inc',
+      'preprocess' => array('template_preprocess_search_results'),
       'template' => 'search-results',
     ),
   );
diff --git a/core/modules/search/tests/modules/search_embedded_form/search_embedded_form.module b/core/modules/search/tests/modules/search_embedded_form/search_embedded_form.module
index edc3cbc..f3c1f52 100644
--- a/core/modules/search/tests/modules/search_embedded_form/search_embedded_form.module
+++ b/core/modules/search/tests/modules/search_embedded_form/search_embedded_form.module
@@ -10,6 +10,16 @@
  */
 
 /**
+ * Implements hook_theme().
+ */
+function search_embedded_form_theme() {
+  $theme['search_result'] = array(
+    'preprocess' => array('search_embedded_form_preprocess_search_result'),
+  );
+  return $theme;
+}
+
+/**
   * Implements hook_menu().
   */
 function search_embedded_form_menu() {
diff --git a/core/modules/shortcut/shortcut.module b/core/modules/shortcut/shortcut.module
index ea58cc4..3cc1804 100644
--- a/core/modules/shortcut/shortcut.module
+++ b/core/modules/shortcut/shortcut.module
@@ -176,6 +176,12 @@ function shortcut_admin_paths() {
  */
 function shortcut_theme() {
   return array(
+    'block__shortcut' => array(
+      'preprocess' => array('shortcut_preprocess_block'),
+    ),
+    'page' => array(
+      'preprocess' => array('shortcut_preprocess_page'),
+    ),
     'shortcut_set_customize' => array(
       'render element' => 'form',
       'file' => 'shortcut.admin.inc',
diff --git a/core/modules/statistics/statistics.module b/core/modules/statistics/statistics.module
index 801742c..61533e0 100644
--- a/core/modules/statistics/statistics.module
+++ b/core/modules/statistics/statistics.module
@@ -83,6 +83,16 @@ function statistics_exit() {
 }
 
 /**
+ * Implements hook_theme().
+ */
+function statistics_theme() {
+  $theme['block__statistics'] = array(
+    'preprocess' => array('statistics_preprocess_block'),
+  );
+  return $theme;
+}
+
+/**
  * Implements hook_permission().
  */
 function statistics_permission() {
diff --git a/core/modules/system/lib/Drupal/system/Tests/Theme/FastTest.php b/core/modules/system/lib/Drupal/system/Tests/Theme/FastTest.php
index 843106e..21742e4 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Theme/FastTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Theme/FastTest.php
@@ -38,6 +38,7 @@ function setUp() {
    * Tests access to user autocompletion and verify the correct results.
    */
   function testUserAutocomplete() {
+    // @todo Replace with injected mock registry in DUTB test.
     $this->drupalLogin($this->account);
     $this->drupalGet('user/autocomplete', array('query' => array('q' => $this->account->name)));
     $this->assertRaw($this->account->name);
diff --git a/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTest.php b/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTest.php
index 5267300..1ad6fa1 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTest.php
@@ -79,6 +79,22 @@ function testPreprocessForSuggestions() {
   }
 
   /**
+   * Ensures suggestion preprocess functions run even for default implementations.
+   *
+   * The theme hook used by this test has its base preprocess function in a
+   * separate file, so this test also ensures that that file is correctly loaded
+   * when needed.
+   */
+  function testSuggestionPreprocessForDefaults() {
+    // Test with both an unprimed and primed theme registry.
+    drupal_theme_rebuild();
+    for ($i = 0; $i < 2; $i++) {
+      $this->drupalGet('theme-test/preprocess-suggestion');
+      $this->assertText('Theme hook implementor=test_theme_theme_test_preprocess__suggestion(). Foo=template_preprocess_theme_test_preprocess', 'Theme hook ran with data available from a preprocess function for the suggested hook.');
+    }
+  }
+
+  /**
    * Ensure page-front template suggestion is added when on front page.
    */
   function testFrontPageThemeSuggestion() {
@@ -192,14 +208,8 @@ function testClassLoading() {
    * Tests drupal_find_theme_templates().
    */
   public function testFindThemeTemplates() {
-    $cache = array();
-
-    // Prime the theme cache.
-    foreach (module_implements('theme') as $module) {
-      _theme_process_registry($cache, $module, 'module', $module, drupal_get_path('module', $module));
-    }
-
-    $templates = drupal_find_theme_templates($cache, '.tpl.php', drupal_get_path('theme', 'test_theme'));
+    $registry = theme_get_registry();
+    $templates = drupal_find_theme_templates($registry, '.tpl.php', drupal_get_path('theme', 'test_theme'));
     $this->assertEqual($templates['node__1']['template'], 'node--1', 'Template node--1.tpl.php was found in test_theme.');
   }
 }
diff --git a/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTestTwig.php b/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTestTwig.php
index 6a9d988..5abb4bc 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTestTwig.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTestTwig.php
@@ -47,13 +47,7 @@ function testTemplateOverride() {
    * Tests drupal_find_theme_templates
    */
   function testFindThemeTemplates() {
-
-    $cache = array();
-
-    // Prime the theme cache
-    foreach (module_implements('theme') as $module) {
-      _theme_process_registry($cache, $module, 'module', $module, drupal_get_path('module', $module));
-    }
+    $cache = theme_get_registry();
 
     // Check for correct content
     // @todo Remove this tests once double engine code is removed
diff --git a/core/modules/system/system.api.php b/core/modules/system/system.api.php
index 7e95a03..d86920d 100644
--- a/core/modules/system/system.api.php
+++ b/core/modules/system/system.api.php
@@ -1806,21 +1806,13 @@ function hook_permission() {
  *   - base hook: A string declaring the base theme hook if this theme
  *     implementation is actually implementing a suggestion for another theme
  *     hook.
- *   - pattern: A regular expression pattern to be used to allow this theme
- *     implementation to have a dynamic name. The convention is to use __ to
- *     differentiate the dynamic portion of the theme. For example, to allow
- *     forums to be themed individually, the pattern might be: 'forum__'. Then,
- *     when the forum is themed, call:
- *     @code
- *     theme(array('forum__' . $tid, 'forum'), $forum)
- *     @endcode
  *   - preprocess functions: A list of functions used to preprocess this data.
  *     Ordinarily this won't be used; it's automatically filled in. By default,
  *     for a module this will be filled in as template_preprocess_HOOK. For
  *     a theme this will be filled in as phptemplate_preprocess and
  *     phptemplate_preprocess_HOOK as well as themename_preprocess and
  *     themename_preprocess_HOOK.
- *   - override preprocess functions: Set to TRUE when a theme does NOT want
+ *   - no preprocess: Set to TRUE when a theme does NOT want
  *     the standard preprocess functions to run. This can be used to give a
  *     theme FULL control over how variables are set. For example, if a theme
  *     wants total control over how certain variables in the page.tpl.php are
@@ -1866,8 +1858,9 @@ function hook_theme($existing, $type, $theme, $path) {
  * Changes here will not be visible until the next cache clear.
  *
  * The $theme_registry array is keyed by theme hook name, and contains the
- * information returned from hook_theme(), as well as additional properties
- * added by _theme_process_registry().
+ * processed information returned from hook_theme().
+ *
+ * @see \Drupal\Core\Theme\Registry::$registry
  *
  * For example:
  * @code
@@ -1890,7 +1883,6 @@ function hook_theme($existing, $type, $theme, $path) {
  *   The entire cache of theme registry information, post-processing.
  *
  * @see hook_theme()
- * @see _theme_process_registry()
  */
 function hook_theme_registry_alter(&$theme_registry) {
   // Kill the next/previous forum topic navigation links.
diff --git a/core/modules/system/system.module b/core/modules/system/system.module
index cdf7b90..d778218 100644
--- a/core/modules/system/system.module
+++ b/core/modules/system/system.module
@@ -133,6 +133,9 @@ function system_help($path, $arg) {
  */
 function system_theme() {
   return array_merge(drupal_common_theme(), array(
+    'block' => array(
+      'preprocess' => array('system_preprocess_block'),
+    ),
     'system_themes_page' => array(
       'variables' => array('theme_groups' => NULL),
       'file' => 'system.admin.inc',
diff --git a/core/modules/system/tests/modules/theme_test/theme_test.inc b/core/modules/system/tests/modules/theme_test/theme_test.inc
index 6cde683..e43df50 100644
--- a/core/modules/system/tests/modules/theme_test/theme_test.inc
+++ b/core/modules/system/tests/modules/theme_test/theme_test.inc
@@ -13,3 +13,22 @@ function theme_theme_test($variables) {
 function template_preprocess_theme_test(&$variables) {
   $variables['foo'] = 'template_preprocess_theme_test';
 }
+
+/**
+ * Returns HTML for the 'theme_test_preprocess' theme hook used by tests.
+ */
+function theme_theme_test_preprocess($variables) {
+  return 'Theme hook implementor=theme_theme_test_preprocess(). Foo=' . $variables['foo'];
+}
+
+/**
+ * Implements hook_preprocess_HOOK() for theme_theme_test_preprocess().
+ *
+ * Despite not having a corresponding theme function for this suggestion, the
+ * specific preprocess function should still be used.
+ */
+function template_preprocess_theme_test_preprocess(&$variables) {
+  $variables['foo'] = 'template_preprocess_theme_test_preprocess';
+}
+
+
diff --git a/core/modules/system/tests/modules/theme_test/theme_test.module b/core/modules/system/tests/modules/theme_test/theme_test.module
index 73ff44b..d5f8cc6 100644
--- a/core/modules/system/tests/modules/theme_test/theme_test.module
+++ b/core/modules/system/tests/modules/theme_test/theme_test.module
@@ -4,9 +4,17 @@
  * Implements hook_theme().
  */
 function theme_test_theme($existing, $type, $theme, $path) {
+  $items['html'] = array(
+    'preprocess' => array('theme_test_preprocess_html'),
+  );
   $items['theme_test'] = array(
     'file' => 'theme_test.inc',
     'variables' => array('foo' => ''),
+    'preprocess' => array('template_preprocess_theme_test'),
+  );
+  $items['theme_test_preprocess'] = array(
+    'file' => 'theme_test.inc',
+    'variables' => array('foo' => ''),
   );
   $items['theme_test_template_test'] = array(
     'template' => 'theme_test.template_test',
@@ -42,6 +50,13 @@ function theme_test_menu() {
     'theme callback' => '_theme_custom_theme',
     'type' => MENU_CALLBACK,
   );
+  $items['theme-test/preprocess-suggestion'] = array(
+    'title' => 'Preprocess suggestion',
+    'page callback' => '_theme_test_preprocess_suggestion',
+    'access callback' => TRUE,
+    'theme callback' => '_theme_custom_theme',
+    'type' => MENU_CALLBACK,
+  );
   $items['theme-test/alter'] = array(
     'title' => 'Suggestion',
     'page callback' => '_theme_test_alter',
@@ -82,19 +97,6 @@ function theme_test_init() {
     // still capable of returning output and theming the page as a whole.
     $GLOBALS['theme_test_output'] = theme('more_link', array('url' => 'user', 'title' => 'Themed output generated in hook_init()'));
   }
-  if (arg(0) == 'user' && arg(1) == 'autocomplete') {
-    // Register a fake registry loading callback. If it gets called by
-    // theme_get_registry(), the registry has not been initialized yet.
-    _theme_registry_callback('_theme_test_load_registry', array());
-  }
-}
-
-/**
- * Fake registry loading callback.
- */
-function _theme_test_load_registry() {
-  print 'registry initialized';
-  return array();
 }
 
 /**
@@ -158,6 +160,13 @@ function _theme_test_suggestion() {
 }
 
 /**
+ * Page callback, calls a theme hook suggestion.
+ */
+function _theme_test_preprocess_suggestion() {
+  return theme(array('theme_test_preprocess__suggestion', 'theme_test_preprocess'), array());
+}
+
+/**
  * Implements hook_preprocess_HOOK() for html.tpl.php.
  */
 function theme_test_preprocess_html(&$variables) {
diff --git a/core/modules/system/tests/themes/test_theme/template.php b/core/modules/system/tests/themes/test_theme/template.php
index 8275818..4d2e947 100644
--- a/core/modules/system/tests/themes/test_theme/template.php
+++ b/core/modules/system/tests/themes/test_theme/template.php
@@ -8,6 +8,13 @@ function test_theme_theme_test__suggestion($variables) {
 }
 
 /**
+ * Tests a theme overriding a default hook with a suggestion.
+ */
+function test_theme_theme_test_preprocess__suggestion($variables) {
+  return 'Theme hook implementor=test_theme_theme_test_preprocess__suggestion(). Foo=' . $variables['foo'];
+}
+
+/**
  * Tests a theme implementing an alter hook.
  *
  * The confusing function name here is due to this being an implementation of
diff --git a/core/modules/taxonomy/taxonomy.module b/core/modules/taxonomy/taxonomy.module
index a15874e..a9a6942 100644
--- a/core/modules/taxonomy/taxonomy.module
+++ b/core/modules/taxonomy/taxonomy.module
@@ -233,6 +233,7 @@ function taxonomy_theme() {
     ),
     'taxonomy_term' => array(
       'render element' => 'elements',
+      'preprocess' => array('template_preprocess_taxonomy_term'),
       'template' => 'taxonomy-term',
     ),
   );
diff --git a/core/modules/toolbar/toolbar.module b/core/modules/toolbar/toolbar.module
index e543326..4dbded4 100644
--- a/core/modules/toolbar/toolbar.module
+++ b/core/modules/toolbar/toolbar.module
@@ -42,6 +42,7 @@ function toolbar_permission() {
 function toolbar_theme($existing, $type, $theme, $path) {
   $items['toolbar'] = array(
     'render element' => 'toolbar',
+    'preprocess' => array('template_preprocess_toolbar'),
     'template' => 'toolbar',
   );
   return $items;
diff --git a/core/modules/user/user.module b/core/modules/user/user.module
index 02f5072..88583ec 100644
--- a/core/modules/user/user.module
+++ b/core/modules/user/user.module
@@ -88,8 +88,12 @@ function user_help($path, $arg) {
  */
 function user_theme() {
   return array(
+    'block__user' => array(
+      'preprocess' => array('user_preprocess_block'),
+    ),
     'user' => array(
       'render element' => 'elements',
+      'preprocess' => array('template_preprocess_user'),
       'template' => 'user',
       'file' => 'user.pages.inc',
     ),
@@ -109,6 +113,8 @@ function user_theme() {
       'variables' => array('signature' => NULL),
     ),
     'username' => array(
+      'preprocess' => array('template_preprocess_username'),
+      'process' => array('template_process_username'),
       'variables' => array('account' => NULL),
     ),
   );
diff --git a/core/modules/views/lib/Drupal/views/Plugin/views/display/DisplayPluginBase.php b/core/modules/views/lib/Drupal/views/Plugin/views/display/DisplayPluginBase.php
index a9d79ba..6851f70 100644
--- a/core/modules/views/lib/Drupal/views/Plugin/views/display/DisplayPluginBase.php
+++ b/core/modules/views/lib/Drupal/views/Plugin/views/display/DisplayPluginBase.php
@@ -7,6 +7,7 @@
 
 namespace Drupal\views\Plugin\views\display;
 
+use Drupal\Core\Theme\Registry;
 use Drupal\views\ViewExecutable;
 use \Drupal\views\Plugin\views\PluginBase;
 
@@ -1765,7 +1766,7 @@ public function buildOptionsForm(&$form, &$form_state) {
               include_once DRUPAL_ROOT . '/' . $theme->owner;
             }
           }
-          $this->theme_registry = _theme_load_registry($theme, $base_theme, $theme_engine);
+          $this->theme_registry = new Registry($theme, $base_theme, $theme_engine);
         }
 
         // If there's a theme engine involved, we also need to know its extension
diff --git a/core/modules/views/tests/views_test_data/views_test_data.module b/core/modules/views/tests/views_test_data/views_test_data.module
index 324a446..742b14a 100644
--- a/core/modules/views/tests/views_test_data/views_test_data.module
+++ b/core/modules/views/tests/views_test_data/views_test_data.module
@@ -8,6 +8,17 @@
 use Drupal\views\ViewExecutable;
 
 /**
+ * Implements hook_theme().
+ */
+function views_test_data_theme() {
+  $theme['views_view_mapping_test'] = array(
+    'render element' => 'element',
+    'preprocess' => array('template_preprocess_views_view_mapping_test'),
+  );
+  return $theme;
+}
+
+/**
  * Implements hook_permission().
  */
 function views_test_data_permission() {
diff --git a/core/modules/views/views.module b/core/modules/views/views.module
index 287e2e0..d1ea0fa 100644
--- a/core/modules/views/views.module
+++ b/core/modules/views/views.module
@@ -82,6 +82,16 @@ function views_pre_render_view_element($element) {
  * Implement hook_theme(). Register views theming functions.
  */
 function views_theme($existing, $type, $theme, $path) {
+  $hooks['comment'] = array(
+    'preprocess' => array('views_preprocess_comment'),
+  );
+  $hooks['html'] = array(
+    'preprocess' => array('views_preprocess_html'),
+  );
+  $hooks['node'] = array(
+    'preprocess' => array('views_preprocess_node'),
+  );
+
   module_load_include('inc', 'views', 'views.theme');
 
   // Some quasi clever array merging here.
diff --git a/core/modules/views/views_ui/views_ui.module b/core/modules/views/views_ui/views_ui.module
index 7552f63..7b10cd9 100644
--- a/core/modules/views/views_ui/views_ui.module
+++ b/core/modules/views/views_ui/views_ui.module
@@ -268,6 +268,10 @@ function views_ui_theme() {
       'render element' => 'element',
       'file' => 'views_ui.theme.inc',
     ),
+
+    'views_view' => array(
+      'preprocess' => array('views_ui_preprocess_views_view'),
+    ),
   );
 }
 
diff --git a/core/themes/bartik/template.php b/core/themes/bartik/template.php
index 9a6c8a9..b0095cf 100644
--- a/core/themes/bartik/template.php
+++ b/core/themes/bartik/template.php
@@ -6,6 +6,27 @@
  */
 
 /**
+ * Implements hook_theme().
+ */
+function bartik_theme() {
+  $theme['block'] = array(
+    'preprocess' => array('bartik_preprocess_block'),
+  );
+  $theme['html'] = array(
+    'preprocess' => array('bartik_preprocess_html'),
+    'process' => array('bartik_process_html'),
+  );
+  $theme['maintenance_page'] = array(
+    'preprocess' => array('bartik_preprocess_maintenance_page'),
+    'process' => array('bartik_process_maintenance_page'),
+  );
+  $theme['page'] = array(
+    'process' => array('bartik_process_page'),
+  );
+  return $theme;
+}
+
+/**
  * Implements hook_preprocess_HOOK() for html.tpl.php.
  *
  * Adds body classes if certain regions have content.
diff --git a/core/themes/seven/template.php b/core/themes/seven/template.php
index d679369..c6d0162 100644
--- a/core/themes/seven/template.php
+++ b/core/themes/seven/template.php
@@ -6,6 +6,25 @@
  */
 
 /**
+ * Implements hook_theme().
+ */
+function seven_theme() {
+  $theme['html'] = array(
+    'preprocess' => array('seven_preprocess_html'),
+  );
+  $theme['install_page'] = array(
+    'preprocess' => array('seven_preprocess_install_page'),
+  );
+  $theme['maintenance_page'] = array(
+    'preprocess' => array('seven_preprocess_maintenance_page'),
+  );
+  $theme['page'] = array(
+    'preprocess' => array('seven_preprocess_page'),
+  );
+  return $theme;
+}
+
+/**
  * Implements hook_preprocess_HOOK() for maintenance-page.tpl.php.
  */
 function seven_preprocess_maintenance_page(&$vars) {