diff --git a/core/includes/module.inc b/core/includes/module.inc
index 7a1c076..724fb46 100644
--- a/core/includes/module.inc
+++ b/core/includes/module.inc
@@ -35,7 +35,7 @@ function module_load_all($bootstrap = FALSE, $reset = FALSE) {
 
   // Unless $boostrap is NULL, load the requested set of modules.
   if (isset($bootstrap) && !$has_run) {
-    $type = $bootstrap ? 'bootstrap' : 'module_enabled';
+    $type = $bootstrap ? 'bootstrap' : 'module';
     foreach (module_list($type) as $module) {
       drupal_load('module', $module);
     }
@@ -58,7 +58,7 @@ function module_load_all($bootstrap = FALSE, $reset = FALSE) {
  *
  * @param string $type
  *   The type of list to return:
- *   - module_enabled: All enabled modules.
+ *   - module: All enabled modules.
  *   - bootstrap: All enabled modules required for bootstrap.
  * @param array $fixed_list
  *   (optional) An array of module names to override the list of modules. This
@@ -74,7 +74,7 @@ function module_load_all($bootstrap = FALSE, $reset = FALSE) {
  *
  * @see module_list_reset()
  */
-function module_list($type = 'module_enabled', array $fixed_list = NULL, $reset = FALSE) {
+function module_list($type = 'module', array $fixed_list = NULL, $reset = FALSE) {
   // This static is only used for $fixed_list. It must not be a drupal_static(),
   // since any call to drupal_static_reset() in unit tests would cause an
   // attempt to retrieve the list of modules from the database (which does not
@@ -121,13 +121,13 @@ function module_list_reset() {
  *
  * @param $type
  *   The type of list to return:
- *   - module_enabled: All enabled modules.
+ *   - module: All enabled modules.
  *   - bootstrap: All enabled modules required for bootstrap.
- *   - theme: All themes.
+ *   - theme: All enabled themes.
  *
  * @return
  *   An associative array of modules or themes, keyed by name. For $type
- *   'bootstrap' and 'module_enabled', the array values equal the keys.
+ *   'bootstrap' and 'module', the array values equal the keys.
  *   For $type 'theme', the array values are objects representing the
  *   respective database row, with the 'info' property already unserialized.
  *
@@ -167,13 +167,13 @@ function system_list($type) {
     $lists['bootstrap'] = array_keys($bootstrap_list);
   }
   // Otherwise build the list for enabled modules and themes.
-  elseif (!isset($lists['module_enabled'])) {
+  elseif (!isset($lists['module'])) {
     if ($cached = cache('bootstrap')->get('system_list')) {
       $lists = $cached->data;
     }
-    else {
+    if (!isset($lists[$type])) {
       $lists = array(
-        'module_enabled' => array(),
+        'module' => array(),
         'theme' => array(),
         'filepaths' => array(),
       );
@@ -186,7 +186,7 @@ function system_list($type) {
       $module_files = state()->get('system.module.files');
       foreach ($enabled_modules as $name => $weight) {
         // Build a list of all enabled modules.
-        $lists['module_enabled'][$name] = $name;
+        $lists['module'][$name] = $name;
         // Build a list of filenames so drupal_get_filename can use it.
         $lists['filepaths'][] = array(
           'type' => 'module',
@@ -197,30 +197,26 @@ function system_list($type) {
 
       // Build a list of themes.
       $enabled_themes = (array) config('system.theme')->get('enabled');
-      // @todo Themes include all themes, including disabled/uninstalled. This
-      //   system.theme.data state will go away entirely as soon as themes have
-      //   a proper installation status.
-      // @see http://drupal.org/node/1067408
-      $theme_data = state()->get('system.theme.data');
-      if (empty($theme_data)) {
-        // @todo: system_list() may be called from _drupal_bootstrap_code() and
+      array_walk($enabled_themes, function (&$value, $key) {
+        $value = 'system.theme.data.' . $key;
+      });
+      $theme_data = (array) state()->getMultiple($enabled_themes);
+      // @todo Remove me?
+      if (0 && empty($theme_data)) {
+        // system_list() may be called from _drupal_bootstrap_code() and
         // module_load_all(), in which case system.module is not loaded yet.
         // Prevent a filesystem scan in drupal_load() and include it directly.
-        // @see http://drupal.org/node/1067408
         require_once DRUPAL_ROOT . '/core/modules/system/system.module';
         $theme_data = system_rebuild_theme_data();
       }
       foreach ($theme_data as $name => $theme) {
-        $theme->status = (int) isset($enabled_themes[$name]);
         $lists['theme'][$name] = $theme;
         // Build a list of filenames so drupal_get_filename can use it.
-        if (isset($enabled_themes[$name])) {
-          $lists['filepaths'][] = array(
-            'type' => 'theme',
-            'name' => $name,
-            'filepath' => $theme->filename,
-          );
-        }
+        $lists['filepaths'][] = array(
+          'type' => 'theme',
+          'name' => $name,
+          'filepath' => $theme->filename,
+        );
       }
       // @todo Move into list_themes(). Read info for a particular requested
       //   theme from state instead.
@@ -267,17 +263,16 @@ function system_list($type) {
 function system_list_reset() {
   drupal_static_reset('system_list');
   drupal_static_reset('system_rebuild_module_data');
+  drupal_static_reset('system_rebuild_theme_data');
   drupal_static_reset('list_themes');
   cache('bootstrap')->deleteMultiple(array('bootstrap_modules', 'system_list'));
   cache()->delete('system_info');
-  // Remove last known theme data state.
-  // This causes system_list() to call system_rebuild_theme_data() on its next
-  // invocation. When enabling a module that implements hook_system_info_alter()
-  // to inject a new (testing) theme or manipulate an existing theme, then that
-  // will cause system_list_reset() to be called, but theme data is not
-  // necessarily rebuilt afterwards.
-  // @todo Obsolete with proper installation status for themes.
-  state()->delete('system.theme.data');
+  // Rebuild theme data to update theme states.
+  // When enabling a module that implements hook_system_info_alter() to inject a
+  // new (testing) theme or manipulate an existing theme, then that will cause
+  // system_list_reset() to be called, but theme data is not necessarily rebuilt
+  // afterwards.
+  system_rebuild_theme_data();
 }
 
 /**
diff --git a/core/includes/theme.inc b/core/includes/theme.inc
index 8a35b6e..3fd144b 100644
--- a/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -55,14 +55,44 @@
  */
 
 /**
+ * Loads a theme.
+ *
+ * @param string $name
+ *   The name of a theme to load.
+ *
+ * @return
+ *   The loaded theme object for $name, or FALSE if the passed $name does not
+ *   exist or is not enabled.
+ */
+function theme_load($name) {
+  $themes = list_themes();
+  if (isset($themes[$name]) && drupal_theme_access($themes[$name])) {
+    return $themes[$name];
+  }
+  return FALSE;
+}
+
+/**
+ * Menu title callback; Returns the sanitized human-readable name of a theme.
+ *
+ * @param $theme
+ *   The theme object to return the title for.
+ *
+ * This needs to be a private function to avoid clashing with a possible
+ * theme_menu_title() theme function by Menu or a potential Menu Title module.
+ */
+function _theme_menu_title($theme) {
+  return check_plain($theme->info['name']);
+}
+
+/**
  * Determines if a theme is available to use.
  *
  * @param $theme
  *   Either the name of a theme or a full theme object.
  *
  * @return
- *   Boolean TRUE if the theme is enabled or is the site administration theme;
- *   FALSE otherwise.
+ *   Boolean TRUE if the theme is enabled; FALSE otherwise.
  */
 function drupal_theme_access($theme) {
   if (is_object($theme)) {
@@ -70,6 +100,9 @@ function drupal_theme_access($theme) {
   }
   else {
     $themes = list_themes();
+    if (!isset($themes[$theme])) {
+      return FALSE;
+    }
     return !empty($themes[$theme]->status);
   }
 }
@@ -689,14 +722,14 @@ function _theme_build_registry($theme, $base_theme, $theme_engine) {
  *     declare this theme as their base theme.
 */
 function list_themes($refresh = FALSE) {
-  $list = &drupal_static(__FUNCTION__, array());
+  $list = &drupal_static(__FUNCTION__);
 
   if ($refresh) {
-    $list = array();
+    $list = NULL;
     system_list_reset();
   }
 
-  if (empty($list)) {
+  if (!isset($list)) {
     $list = array();
     $themes = array();
     // Extract from the database only when it is available.
@@ -1460,29 +1493,134 @@ function theme_render_template($template_file, $variables) {
 /**
  * Enable a given list of themes.
  *
- * @param $theme_list
- *   An array of theme names.
+ * @param array $theme_list
+ *   An array of theme names to enable.
+ * @param bool $enable_dependencies
+ *   If TRUE, dependencies will automatically be added and enabled in the
+ *   correct order. This incurs a significant performance cost, so use FALSE
+ *   if you know $theme_list is already complete and in the correct order.
+ *
+ * @todo D8: This is almost identical to module_enable() now. Only differences:
+ *   - There is no required default .theme PHP file to load.
+ *   - There is no schema to install.
+ *   - System list, state, theme list, and static resets slightly differ.
+ * @todo hook_themes_installed(), hook_themes_enabled(), etc are not invoked in
+ *   themes, since they are excluded from the hook system.
  */
-function theme_enable($theme_list) {
-  drupal_clear_css_cache();
+function theme_enable($theme_list, $enable_dependencies = TRUE) {
+  if ($enable_dependencies) {
+    // Get all theme data so we can find dependencies and sort.
+    $theme_data = system_rebuild_theme_data();
+    // Create an associative array with weights as values.
+    $theme_list = array_flip(array_values($theme_list));
+
+    while (list($theme) = each($theme_list)) {
+      if (!isset($theme_data[$theme])) {
+        // This theme is not found in the filesystem, abort.
+        return FALSE;
+      }
+      if ($theme_data[$theme]->status) {
+        // Skip already enabled themes.
+        unset($theme_list[$theme]);
+        continue;
+      }
+      $theme_list[$theme] = $theme_data[$theme]->sort;
+
+      // Add dependencies to the list, with a placeholder weight.
+      // The new themes will be processed as the while loop continues.
+      foreach (array_keys($theme_data[$theme]->requires) as $dependency) {
+        if (!isset($theme_list[$dependency])) {
+          $theme_list[$dependency] = 0;
+        }
+      }
+    }
+
+    if (!$theme_list) {
+      // Nothing to do. All themes already enabled.
+      return TRUE;
+    }
+
+    // Sort the theme list by pre-calculated weights.
+    arsort($theme_list);
+    $theme_list = array_keys($theme_list);
+  }
+
+  // Required for schema version checks and constants.
+  include_once DRUPAL_ROOT . '/core/includes/install.inc';
+
+  $themes_installed = array();
+  $themes_enabled = array();
+  $schema_store = drupal_container()->get('keyvalue')->get('system.schema');
   $theme_config = config('system.theme');
-  $disabled_themes = config('system.theme.disabled');
+  $disabled_config = config('system.theme.disabled');
   foreach ($theme_list as $key) {
-    // The value is not used; the weight is ignored for themes currently.
-    $theme_config->set("enabled.$key", 0);
-    $disabled_themes->clear($key);
-    // Install default configuration of the theme.
-    config_install_default_config('theme', $key);
+    // Only process themes that are not already enabled.
+    $enabled = $theme_config->get("enabled.$key") !== NULL;
+    if (!$enabled) {
+      // The value is not used; the weight is ignored for themes currently.
+      $theme_config
+        ->set("enabled.$key", 0)
+        ->save();
+      $disabled_config
+        ->clear($key)
+        ->save();
+
+      // Refresh the system list to include it.
+      system_list_reset();
+      // Refresh the schema to include it.
+      drupal_get_schema(NULL, TRUE);
+      // Update the theme registry to include it.
+      list_themes(TRUE);
+      drupal_theme_rebuild();
+
+      // Allow modules to react prior to the installation of a theme.
+      module_invoke_all('themes_preinstall', array($key));
+
+      // Now install the theme if necessary.
+      if (drupal_get_installed_schema_version($key, TRUE) == SCHEMA_UNINSTALLED) {
+        $version = SCHEMA_INSTALLED;
+
+        // Install default configuration of the theme.
+        config_install_default_config('theme', $key);
+
+        drupal_set_installed_schema_version($key, $version);
+        // Allow the theme to perform install tasks.
+        $function = $key . '_install';
+        if (function_exists($function)) {
+          $function();
+        }
+        // Record the fact that it was installed.
+        $themes_installed[] = $key;
+        watchdog('system', '%theme theme installed.', array('%theme' => $key), WATCHDOG_INFO);
+      }
+
+      // Allow modules to react prior to the enabling of a theme.
+      module_invoke_all('themes_preenable', array($key));
+
+      // Enable the theme.
+      $function = $key . '_enable';
+      if (function_exists($function)) {
+        $function();
+      }
+
+      // Record the fact that it was enabled.
+      $themes_enabled[] = $key;
+      watchdog('system', '%theme theme enabled.', array('%theme' => $key), WATCHDOG_INFO);
+    }
   }
-  $theme_config->save();
-  $disabled_themes->save();
 
-  list_themes(TRUE);
-  menu_router_rebuild();
-  drupal_theme_rebuild();
+  // If any themes were newly installed, invoke hook_themes_installed().
+  if (!empty($themes_installed)) {
+    module_invoke_all('themes_installed', $themes_installed);
+  }
 
-  // Invoke hook_themes_enabled() after the themes have been enabled.
-  module_invoke_all('themes_enabled', $theme_list);
+  // If any themes were newly enabled, invoke hook_themes_enabled().
+  if (!empty($themes_enabled)) {
+    module_invoke_all('themes_enabled', $themes_enabled);
+  }
+
+  drupal_clear_css_cache();
+  return TRUE;
 }
 
 /**
@@ -1503,17 +1641,18 @@ function theme_disable($theme_list) {
   drupal_clear_css_cache();
 
   $theme_config = config('system.theme');
-  $disabled_themes = config('system.theme.disabled');
+  $disabled_config = config('system.theme.disabled');
   foreach ($theme_list as $key) {
     // The value is not used; the weight is ignored for themes currently.
-    $theme_config->clear("enabled.$key");
-    $disabled_themes->set($key, 0);
+    $disabled_config
+      ->set($key, 0)
+      ->save();
+    $theme_config
+      ->clear("enabled.$key")
+      ->save();
   }
-  $theme_config->save();
-  $disabled_themes->save();
 
   list_themes(TRUE);
-  menu_router_rebuild();
   drupal_theme_rebuild();
 
   // Invoke hook_themes_disabled after the themes have been disabled.
diff --git a/core/includes/theme.maintenance.inc b/core/includes/theme.maintenance.inc
index 4b3e80c..7513cb8 100644
--- a/core/includes/theme.maintenance.inc
+++ b/core/includes/theme.maintenance.inc
@@ -17,11 +17,7 @@
 function _drupal_maintenance_theme() {
   global $theme, $theme_key, $conf;
 
-  // If $theme is already set, assume the others are set too, and do nothing.
-  if (isset($theme)) {
-    return;
-  }
-
+  require_once DRUPAL_ROOT . '/core/includes/database.inc';
   require_once DRUPAL_ROOT . '/' . variable_get('path_inc', 'core/includes/path.inc');
   require_once DRUPAL_ROOT . '/core/includes/theme.inc';
   require_once DRUPAL_ROOT . '/core/includes/common.inc';
@@ -35,13 +31,6 @@ function _drupal_maintenance_theme() {
     $custom_theme = (isset($conf['maintenance_theme']) ? $conf['maintenance_theme'] : 'seven');
   }
   else {
-    // The bootstrap was not complete. So we are operating in a crippled
-    // environment, we need to bootstrap just enough to allow hook invocations
-    // to work. See _drupal_log_error().
-    if (!class_exists('Drupal\Core\Database\Database', FALSE)) {
-      require_once DRUPAL_ROOT . '/core/includes/database.inc';
-    }
-
     // We use the default theme as the maintenance theme. If a default theme
     // isn't specified in the database or in settings.php, we use Bartik.
     // @todo Should use the actual default theme configured, but that depends on
@@ -61,6 +50,9 @@ function _drupal_maintenance_theme() {
   }
 
   $themes = list_themes();
+  if (empty($themes) || !isset($themes[$custom_theme])) {
+    $themes = _system_rebuild_theme_data();
+  }
 
   // list_themes() triggers a drupal_alter() in maintenance mode, but we can't
   // let themes alter the .info data until we know a theme's base themes. So
diff --git a/core/includes/update.inc b/core/includes/update.inc
index 299a81c..655b23f 100644
--- a/core/includes/update.inc
+++ b/core/includes/update.inc
@@ -276,7 +276,7 @@ function update_prepare_d8_bootstrap() {
       // Update the environment for the language bootstrap if needed.
       update_prepare_d8_language();
       // Prime the classloader.
-      system_list('module_enabled');
+      system_list('module');
 
       // Change language column to langcode in url_alias.
       if (db_table_exists('url_alias') && db_field_exists('url_alias', 'language')) {
diff --git a/core/lib/Drupal/Core/ExceptionController.php b/core/lib/Drupal/Core/ExceptionController.php
index 92d4454..a2d82b1 100644
--- a/core/lib/Drupal/Core/ExceptionController.php
+++ b/core/lib/Drupal/Core/ExceptionController.php
@@ -267,9 +267,10 @@ public function on500Html(FlattenException $exception, Request $request) {
       drupal_set_message(t('%type: !message in %function (line %line of %file).', $error), $class);
     }
 
-    drupal_set_title(t('Error'));
     // We fallback to a maintenance page at this point, because the page
     // generation itself can generate errors.
+    drupal_maintenance_theme();
+    drupal_set_title(t('Error'));
     $output = theme('maintenance_page', array('content' => t('The website has encountered an error. Please try again later.')));
 
     $response = new Response($output, 500);
diff --git a/core/modules/block/block.admin.inc b/core/modules/block/block.admin.inc
index ffa0b0b..e51416b 100644
--- a/core/modules/block/block.admin.inc
+++ b/core/modules/block/block.admin.inc
@@ -33,6 +33,9 @@ function block_admin_display($theme = NULL) {
     // If theme is not specifically set, rehash for the current theme.
     $theme = $theme_key;
   }
+  elseif (is_object($theme)) {
+    $theme = $theme->name;
+  }
 
   // Fetch and sort blocks.
   $blocks = block_admin_display_prepare_blocks($theme);
diff --git a/core/modules/block/block.module b/core/modules/block/block.module
index 26bcb25..0216f27 100644
--- a/core/modules/block/block.module
+++ b/core/modules/block/block.module
@@ -62,11 +62,21 @@ function block_help($path, $arg) {
     case 'admin/structure/block/add':
       return '<p>' . t('Use this page to create a new custom block.') . '</p>';
   }
-  if ($arg[0] == 'admin' && $arg[1] == 'structure' && $arg['2'] == 'block' && (empty($arg[3]) || $arg[3] == 'list')) {
-    $demo_theme = !empty($arg[4]) ? $arg[4] : variable_get('theme_default', 'stark');
-    $themes = list_themes();
+
+  if ($arg[0] === 'admin' && $arg[1] === 'structure' && $arg['2'] === 'block' && (empty($arg[3]) || $arg[3] === 'list')) {
     $output = '<p>' . t('This page provides a drag-and-drop interface for assigning a block to a region, and for controlling the order of blocks within regions. Since not all themes implement the same regions, or display regions in the same way, blocks are positioned on a per-theme basis. Remember that your changes will not be saved until you click the <em>Save blocks</em> button at the bottom of the page. Click the <em>configure</em> link next to each block to configure its specific title and visibility settings.') . '</p>';
-    $output .= '<p>' . l(t('Demonstrate block regions (@theme)', array('@theme' => $themes[$demo_theme]->info['name'])), 'admin/structure/block/demo/' . $demo_theme) . '</p>';
+    if (!empty($arg[4])) {
+      if (drupal_theme_access($arg[4])) {
+        $demo_theme = $arg[4];
+      }
+    }
+    else {
+      $demo_theme = variable_get('theme_default', 'stark');
+    }
+    if (isset($demo_theme)) {
+      $themes = list_themes();
+      $output .= '<p>' . l(t('Demonstrate block regions (@theme)', array('@theme' => $themes[$demo_theme]->info['name'])), 'admin/structure/block/demo/' . $demo_theme) . '</p>';
+    }
     return $output;
   }
 }
@@ -103,12 +113,10 @@ function block_permission() {
  * Implements hook_menu().
  */
 function block_menu() {
-  $default_theme = variable_get('theme_default', 'stark');
   $items['admin/structure/block'] = array(
     'title' => 'Blocks',
     'description' => 'Configure what block content appears in your site\'s sidebars and other regions.',
     'page callback' => 'block_admin_display',
-    'page arguments' => array($default_theme),
     'access arguments' => array('administer blocks'),
     'file' => 'block.admin.inc',
   );
@@ -141,39 +149,96 @@ function block_menu() {
     'type' => MENU_LOCAL_ACTION,
     'file' => 'block.admin.inc',
   );
-  foreach (list_themes() as $key => $theme) {
-    $items['admin/structure/block/list/' . $key] = array(
-      'title' => check_plain($theme->info['name']),
-      'page arguments' => array($key),
-      'type' => $key == $default_theme ? MENU_DEFAULT_LOCAL_TASK : MENU_LOCAL_TASK,
-      'weight' => $key == $default_theme ? -10 : 0,
-      'access callback' => '_block_themes_access',
-      'access arguments' => array($key),
-      'file' => 'block.admin.inc',
-    );
-    if ($key != $default_theme) {
-      $items['admin/structure/block/list/' . $key . '/add'] = array(
-        'title' => 'Add block',
-        'page callback' => 'drupal_get_form',
-        'page arguments' => array('block_add_block_form'),
-        'access arguments' => array('administer blocks'),
-        'type' => MENU_LOCAL_ACTION,
-        'file' => 'block.admin.inc',
+
+  $items['admin/structure/block/list/default'] = array(
+    'title' => 'Default theme blocks',
+    'type' => MENU_DEFAULT_LOCAL_TASK,
+    'weight' => -10,
+  );
+  $items['admin/structure/block/list/%theme'] = array(
+    'title' => 'Theme blocks',
+    'title callback' => '_theme_menu_title',
+    'title arguments' => array(4),
+    'page callback' => 'block_admin_display',
+    'page arguments' => array(4),
+    'type' => MENU_LOCAL_TASK,
+    'access callback' => '_block_themes_access',
+    'access arguments' => array(4),
+    'file' => 'block.admin.inc',
+  );
+  $items['admin/structure/block/list/%theme/add'] = array(
+    'title' => 'Add block',
+    'page callback' => 'drupal_get_form',
+    'page arguments' => array('block_add_block_form'),
+    'access arguments' => array('administer blocks'),
+    'type' => MENU_LOCAL_ACTION,
+    'file' => 'block.admin.inc',
+  );
+  $items['admin/structure/block/demo/%theme'] = array(
+    'title' => 'Theme block region demo',
+    'title callback' => '_theme_menu_title',
+    'title arguments' => array(4),
+    'page callback' => 'block_admin_demo',
+    'page arguments' => array(4),
+    'type' => MENU_CALLBACK,
+    'access callback' => '_block_themes_access',
+    'access arguments' => array(4),
+    'theme callback' => '_block_custom_theme',
+    'theme arguments' => array(4),
+    'file' => 'block.admin.inc',
+  );
+  return $items;
+}
+
+/**
+ * Implements hook_menu_local_tasks_alter().
+ */
+function block_menu_local_tasks_alter(&$data, $router_item, $root_path) {
+  if ($root_path === 'admin/structure/block' || $root_path === 'admin/structure/block/list/%') {
+    $tabs = &$data['tabs'][0]['output'];
+    // Determine the currently selected tab, if any.
+    $selected_index = -1;
+    $selected_id = '';
+    foreach ($tabs as $index => &$tab) {
+      if (isset($tab['#link']['path']) && $tab['#link']['path'] == 'admin/structure/block/list/%') {
+        $selected_index = $index;
+        $selected_id = $router_item['original_map'][4];
+        break;
+      }
+    }
+    // Expand the dynamic %theme argument into a tab for each theme.
+    $themes = list_themes();
+    $default_theme = variable_get('theme_default', 'stark');
+    foreach ($themes as $theme) {
+      // The default theme is always exposed as default local task already.
+      if ($theme->name === $default_theme) {
+        $tabs[0]['#link']['title'] = $theme->info['name'];
+        continue;
+      }
+      // If the current page is the active tab registered in hook_menu(), then
+      // the menu router item with the dynamic argument will be exposed already.
+      // We must not duplicate that tab, but in order to ensure that all of our
+      // tabs appear in a consistent order when switching between tabs, we need
+      // to re-inject it.
+      if ($theme->name === $selected_id) {
+        $tabs[$selected_index]['#link']['title'] = $theme->info['name'];
+        $tabs[] = $tabs[$selected_index];
+        unset($tabs[$selected_index]);
+        continue;
+      }
+      $tabs[] = array(
+        '#theme' => 'menu_local_task',
+        '#link' => array(
+          'title' => $theme->info['name'],
+          'href' => 'admin/structure/block/list/' . $theme->name,
+          'localized_options' => array('html' => FALSE),
+        ),
       );
     }
-    $items['admin/structure/block/demo/' . $key] = array(
-      'title' => check_plain($theme->info['name']),
-      'page callback' => 'block_admin_demo',
-      'page arguments' => array($key),
-      'type' => MENU_CALLBACK,
-      'access callback' => '_block_themes_access',
-      'access arguments' => array($key),
-      'theme callback' => '_block_custom_theme',
-      'theme arguments' => array($key),
-      'file' => 'block.admin.inc',
-    );
+    if (count($themes) > 1) {
+      $data['tabs'][0]['count']++;
+    }
   }
-  return $items;
 }
 
 /**
@@ -208,7 +273,9 @@ function _block_themes_access($theme) {
 function _block_custom_theme($theme = NULL) {
   // We return exactly what was passed in, to guarantee that the page will
   // always be displayed using the theme whose blocks are being configured.
-  return $theme;
+  if (isset($theme->name)) {
+    return $theme->name;
+  }
 }
 
 /**
@@ -273,7 +340,7 @@ function block_page_build(&$page) {
   $all_regions = system_region_list($theme);
 
   $item = menu_get_item();
-  if ($item['path'] != 'admin/structure/block/demo/' . $theme) {
+  if ($item['path'] != 'admin/structure/block/demo/%') {
     // Load all region content assigned via blocks.
     foreach (array_keys($all_regions) as $region) {
       // Assign blocks to region.
@@ -293,8 +360,7 @@ function block_page_build(&$page) {
   }
   else {
     // Append region description if we are rendering the regions demo page.
-    $item = menu_get_item();
-    if ($item['path'] == 'admin/structure/block/demo/' . $theme) {
+    if ($item['path'] == 'admin/structure/block/demo/%') {
       $visible_regions = array_keys(system_region_list($theme, REGIONS_VISIBLE));
       foreach ($visible_regions as $region) {
         $description = '<div class="block-region">' . $all_regions[$region] . '</div>';
@@ -412,6 +478,10 @@ function _block_rehash($theme = NULL) {
   }
   $regions = system_region_list($theme);
 
+  if (is_object($theme)) {
+    $theme = $theme->name;
+  }
+
   // These are the blocks the function will return.
   $blocks = array();
   // These are the blocks defined by code and modified by the database.
diff --git a/core/modules/block/lib/Drupal/block/Tests/BlockAdminThemeTest.php b/core/modules/block/lib/Drupal/block/Tests/BlockAdminThemeTest.php
index c3a220f..ddee5dc 100644
--- a/core/modules/block/lib/Drupal/block/Tests/BlockAdminThemeTest.php
+++ b/core/modules/block/lib/Drupal/block/Tests/BlockAdminThemeTest.php
@@ -37,9 +37,9 @@ function testAdminTheme() {
     $admin_user = $this->drupalCreateUser(array('administer blocks', 'administer themes'));
     $this->drupalLogin($admin_user);
 
-    // Ensure that access to block admin page is denied when theme is disabled.
+    // Ensure the block admin page cannot be accessed for a disabled theme.
     $this->drupalGet('admin/structure/block/list/bartik');
-    $this->assertResponse(403);
+    $this->assertResponse(404);
 
     // Enable admin theme and confirm that tab is accessible.
     theme_enable(array('bartik'));
diff --git a/core/modules/block/lib/Drupal/block/Tests/BlockHiddenRegionTest.php b/core/modules/block/lib/Drupal/block/Tests/BlockHiddenRegionTest.php
index af5acb9..2bf01ed 100644
--- a/core/modules/block/lib/Drupal/block/Tests/BlockHiddenRegionTest.php
+++ b/core/modules/block/lib/Drupal/block/Tests/BlockHiddenRegionTest.php
@@ -65,7 +65,7 @@ function testBlockNotInHiddenRegion() {
     $theme = 'block_test_theme';
     theme_enable(array($theme));
     variable_set('theme_default', $theme);
-    menu_router_rebuild();
+    menu_cache_clear_all();
 
     // Ensure that "block_test_theme" is set as the default theme.
     $this->drupalGet('admin/structure/block');
diff --git a/core/modules/shortcut/lib/Drupal/shortcut/Tests/ShortcutLinksTest.php b/core/modules/shortcut/lib/Drupal/shortcut/Tests/ShortcutLinksTest.php
index ccf9834..a7bcd7c 100644
--- a/core/modules/shortcut/lib/Drupal/shortcut/Tests/ShortcutLinksTest.php
+++ b/core/modules/shortcut/lib/Drupal/shortcut/Tests/ShortcutLinksTest.php
@@ -122,6 +122,7 @@ function testShortcutLinkDelete() {
    */
   function testNoShortcutLink() {
     // Change to a theme that displays shortcuts.
+    theme_enable(array('seven'));
     variable_set('theme_default', 'seven');
 
     $this->drupalGet('page-that-does-not-exist');
diff --git a/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php b/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php
index 93cdbf2..ec66409 100644
--- a/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php
+++ b/core/modules/simpletest/lib/Drupal/simpletest/TestBase.php
@@ -883,11 +883,6 @@ protected function prepareEnvironment() {
     unset($GLOBALS['theme_key']);
     unset($GLOBALS['theme']);
 
-    // Re-initialize the theme to ensure that tests do not see an inconsistent
-    // behavior when calling functions that would initialize the theme if it has
-    // not been initialized yet.
-    drupal_theme_initialize();
-
     // Log fatal errors.
     ini_set('log_errors', 1);
     ini_set('error_log', $this->public_files_directory . '/error.log');
diff --git a/core/modules/simpletest/lib/Drupal/simpletest/UnitTestBase.php b/core/modules/simpletest/lib/Drupal/simpletest/UnitTestBase.php
index 648aa42..0b112ea 100644
--- a/core/modules/simpletest/lib/Drupal/simpletest/UnitTestBase.php
+++ b/core/modules/simpletest/lib/Drupal/simpletest/UnitTestBase.php
@@ -70,6 +70,11 @@ protected function setUp() {
       return FALSE;
     }
 
+    // Re-initialize the theme to ensure that tests do not see an inconsistent
+    // behavior when calling functions that would initialize the theme if it has
+    // not been initialized yet.
+    drupal_theme_initialize();
+
     // Set user agent to be consistent with WebTestBase.
     $_SERVER['HTTP_USER_AGENT'] = $this->databasePrefix;
 
diff --git a/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php b/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
index 001e521..e5f43ab 100644
--- a/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
+++ b/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
@@ -760,6 +760,11 @@ protected function setUp() {
       $this->rebuildContainer();
     }
 
+    // Re-initialize the theme to ensure that tests do not see an inconsistent
+    // behavior when calling functions that would initialize the theme if it has
+    // not been initialized yet.
+    drupal_theme_initialize();
+
     // Reset/rebuild all data structures after enabling the modules.
     $this->resetAll();
 
diff --git a/core/modules/system/lib/Drupal/system/Tests/Batch/PageTest.php b/core/modules/system/lib/Drupal/system/Tests/Batch/PageTest.php
index db1d671..4788705 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Batch/PageTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Batch/PageTest.php
@@ -35,8 +35,8 @@ public static function getInfo() {
   function testBatchProgressPageTheme() {
     // Make sure that the page which starts the batch (an administrative page)
     // is using a different theme than would normally be used by the batch API.
+    theme_enable(array('bartik', 'seven'));
     variable_set('theme_default', 'bartik');
-    theme_enable(array('seven'));
     variable_set('admin_theme', 'seven');
     // Log in as an administrator who can see the administrative theme.
     $admin_user = $this->drupalCreateUser(array('view the administration theme'));
diff --git a/core/modules/system/lib/Drupal/system/Tests/System/ThemeTest.php b/core/modules/system/lib/Drupal/system/Tests/System/ThemeTest.php
index 111e6b7..d0d0896 100644
--- a/core/modules/system/lib/Drupal/system/Tests/System/ThemeTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/System/ThemeTest.php
@@ -206,7 +206,7 @@ function testAdministrationTheme() {
     $this->assertRaw('core/themes/stark', 'Site default theme used on the add content page.');
 
     // Reset to the default theme settings.
-    variable_set('theme_default', 'bartik');
+    variable_set('theme_default', 'stark');
     $edit = array(
       'admin_theme' => '0',
       'node_admin_theme' => FALSE,
@@ -214,10 +214,10 @@ function testAdministrationTheme() {
     $this->drupalPost('admin/appearance', $edit, t('Save configuration'));
 
     $this->drupalGet('admin');
-    $this->assertRaw('core/themes/bartik', 'Site default theme used on administration page.');
+    $this->assertRaw('core/themes/stark', 'Site default theme used on administration page.');
 
     $this->drupalGet('node/add');
-    $this->assertRaw('core/themes/bartik', 'Site default theme used on the add content page.');
+    $this->assertRaw('core/themes/stark', 'Site default theme used on the add content page.');
   }
 
   /**
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 f1308c5..3e214c6 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Theme/ThemeTest.php
@@ -141,18 +141,20 @@ function testTemplateOverride() {
    * Test the list_themes() function.
    */
   function testListThemes() {
+    theme_enable(array('test_subtheme'));
     $themes = list_themes();
     // Check if drupal_theme_access() retrieves enabled themes properly from list_themes().
     $this->assertTrue(drupal_theme_access('test_theme'), 'Enabled theme detected');
-    // Check if list_themes() returns disabled themes.
-    $this->assertTrue(array_key_exists('test_basetheme', $themes), 'Disabled theme detected');
+
     // Check for base theme and subtheme lists.
     $base_theme_list = array('test_basetheme' => 'Theme test base theme');
     $sub_theme_list = array('test_subtheme' => 'Theme test subtheme');
     $this->assertIdentical($themes['test_basetheme']->sub_themes, $sub_theme_list, 'Base theme\'s object includes list of subthemes.');
     $this->assertIdentical($themes['test_subtheme']->base_themes, $base_theme_list, 'Subtheme\'s object includes list of base themes.');
+
     // Check for theme engine in subtheme.
     $this->assertIdentical($themes['test_subtheme']->engine, 'phptemplate', 'Subtheme\'s object includes the theme engine.');
+
     // Check for theme engine prefix.
     $this->assertIdentical($themes['test_basetheme']->prefix, 'phptemplate', 'Base theme\'s object includes the theme engine prefix.');
     $this->assertIdentical($themes['test_subtheme']->prefix, 'phptemplate', 'Subtheme\'s object includes the theme engine prefix.');
@@ -162,6 +164,7 @@ function testListThemes() {
    * Test the theme_get_setting() function.
    */
   function testThemeGetSetting() {
+    theme_enable(array('test_subtheme'));
     $GLOBALS['theme_key'] = 'test_theme';
     $this->assertIdentical(theme_get_setting('theme_test_setting'), 'default value', 'theme_get_setting() uses the default theme automatically.');
     $this->assertNotEqual(theme_get_setting('subtheme_override', 'test_basetheme'), theme_get_setting('subtheme_override', 'test_subtheme'), 'Base theme\'s default settings values can be overridden by subtheme.');
diff --git a/core/modules/system/system.admin.inc b/core/modules/system/system.admin.inc
index 46b4b9d..9570b5a 100644
--- a/core/modules/system/system.admin.inc
+++ b/core/modules/system/system.admin.inc
@@ -119,8 +119,9 @@ function system_themes_page() {
   uasort($themes, 'system_sort_modules_by_info_name');
 
   $theme_default = variable_get('theme_default', 'stark');
-  $theme_groups  = array();
+  $theme_groups = array('enabled' => array(), 'disabled' => array());
   $admin_theme = variable_get('admin_theme', 0);
+  $admin_theme_options = array();
 
   foreach ($themes as &$theme) {
     if (!empty($theme->info['hidden'])) {
@@ -281,12 +282,10 @@ function system_themes_admin_form_submit($form, &$form_state) {
 function system_theme_enable() {
   if (isset($_REQUEST['theme']) && isset($_REQUEST['token']) && drupal_valid_token($_REQUEST['token'], 'system-theme-operation-link')) {
     $theme = $_REQUEST['theme'];
-    // Get current list of themes.
-    $themes = list_themes();
-
     // Check if the specified theme is one recognized by the system.
-    if (!empty($themes[$theme])) {
+    if (drupal_get_path('theme', $theme)) {
       theme_enable(array($theme));
+      $themes = list_themes();
       drupal_set_message(t('The %theme theme has been enabled.', array('%theme' => $themes[$theme]->info['name'])));
     }
     else {
@@ -331,26 +330,17 @@ function system_theme_disable() {
 function system_theme_default() {
   if (isset($_REQUEST['theme']) && isset($_REQUEST['token']) && drupal_valid_token($_REQUEST['token'], 'system-theme-operation-link')) {
     $theme = $_REQUEST['theme'];
-    // Get current list of themes.
-    $themes = list_themes();
-
     // Check if the specified theme is one recognized by the system.
-    if (!empty($themes[$theme])) {
+    if (drupal_get_path('theme', $theme)) {
+      $themes = list_themes();
       // Enable the theme if it is currently disabled.
-      if (empty($themes[$theme]->status)) {
-       theme_enable(array($theme));
+      if (!isset($themes[$theme])) {
+        theme_enable(array($theme));
+        $themes = list_themes();
       }
       // Set the default theme.
       variable_set('theme_default', $theme);
 
-      // Rebuild the menu. This duplicates the menu_router_rebuild() in
-      // theme_enable(). However, modules must know the current default theme in
-      // order to use this information in hook_menu() or hook_menu_alter()
-      // implementations, and doing the variable_set() before the theme_enable()
-      // could result in a race condition where the theme is default but not
-      // enabled.
-      menu_router_rebuild();
-
       // The status message depends on whether an admin theme is currently in use:
       // a value of 0 means the admin theme is set to be the default theme.
       $admin_theme = variable_get('admin_theme', 0);
@@ -375,14 +365,17 @@ function system_theme_default() {
 /**
  * Form builder; display theme configuration for entire site and individual themes.
  *
- * @param $key
- *   A theme name.
+ * @param $theme
+ *   (optional) A theme object.
+ *
  * @return
  *   The form structure.
+ *
  * @ingroup forms
  * @see system_theme_settings_submit()
  */
-function system_theme_settings($form, &$form_state, $key = '') {
+function system_theme_settings($form, &$form_state, $theme = NULL) {
+  $key = !empty($theme) ? $theme->name : '';
   // Default settings are defined in theme_get_setting() in includes/theme.inc
   if ($key) {
     $var = 'theme_' . $key . '_settings';
diff --git a/core/modules/system/system.module b/core/modules/system/system.module
index 1492cf0..1a92ef1 100644
--- a/core/modules/system/system.module
+++ b/core/modules/system/system.module
@@ -686,19 +686,18 @@ function system_menu() {
   $items['admin/appearance/settings/global'] = array(
     'title' => 'Global settings',
     'type' => MENU_DEFAULT_LOCAL_TASK,
-    'weight' => -1,
+    'weight' => -10,
+  );
+  $items['admin/appearance/settings/%theme'] = array(
+    'title' => 'Theme settings',
+    'title callback' => '_theme_menu_title',
+    'title arguments' => array(3),
+    'page arguments' => array('system_theme_settings', 3),
+    'access callback' => '_system_themes_access',
+    'access arguments' => array(3),
+    'type' => MENU_LOCAL_TASK,
+    'file' => 'system.admin.inc',
   );
-
-  foreach (list_themes() as $key => $theme) {
-    $items['admin/appearance/settings/' . $theme->name] = array(
-      'title' => $theme->info['name'],
-      'page arguments' => array('system_theme_settings', $theme->name),
-      'type' => MENU_LOCAL_TASK,
-      'access callback' => '_system_themes_access',
-      'access arguments' => array($key),
-      'file' => 'system.admin.inc',
-    );
-  }
 
   // Modules.
   $items['admin/modules'] = array(
@@ -1033,6 +1032,51 @@ function system_menu() {
 }
 
 /**
+ * Implements hook_menu_local_tasks_alter().
+ */
+function system_menu_local_tasks_alter(&$data, $router_item, $root_path) {
+  if ($root_path === 'admin/appearance/settings' || $root_path === 'admin/appearance/settings/%') {
+    $tabs = &$data['tabs'][1]['output'];
+    // Determine the currently selected tab, if any.
+    $selected_index = -1;
+    $selected_id = '';
+    foreach ($tabs as $index => &$tab) {
+      if (isset($tab['#link']['path']) && $tab['#link']['path'] == 'admin/appearance/settings/%') {
+        $selected_index = $index;
+        $selected_id = $router_item['original_map'][3];
+        break;
+      }
+    }
+    // Expand the dynamic %theme argument into a tab for each theme.
+    $themes = list_themes();
+    foreach ($themes as $theme) {
+      // If the current page is the active tab registered in hook_menu(), then
+      // the menu router item with the dynamic argument will be exposed already.
+      // We must not duplicate that tab, but in order to ensure that all of our
+      // tabs appear in a consistent order when switching between tabs, we need
+      // to re-inject it.
+      if ($theme->name === $selected_id) {
+        $tabs[$selected_index]['#link']['title'] = $theme->info['name'];
+        $tabs[] = $tabs[$selected_index];
+        unset($tabs[$selected_index]);
+        continue;
+      }
+      $tabs[] = array(
+        '#theme' => 'menu_local_task',
+        '#link' => array(
+          'title' => $theme->info['name'],
+          'href' => 'admin/appearance/settings/' . $theme->name,
+          'localized_options' => array('html' => FALSE),
+        ),
+      );
+    }
+    if (count($themes) > 1) {
+      $data['tabs'][1]['count']++;
+    }
+  }
+}
+
+/**
  * Theme callback for the default batch page.
  */
 function _system_batch_theme() {
@@ -2698,6 +2742,7 @@ function system_get_info($type, $name = NULL) {
     }
   }
   else {
+    // @todo Migrate theme info to ModuleInfo implementation.
     $list = system_list($type);
     foreach ($list as $shortname => $item) {
       if (!empty($item->status)) {
@@ -2723,6 +2768,8 @@ function system_get_info($type, $name = NULL) {
  *   returned.
  *
  * @see Drupal\Core\Utility\ModuleInfo
+ *
+ * @todo Merge this helper entirely into system_get_info()?
  */
 function system_get_module_info($property) {
   static $info;
@@ -2935,6 +2982,9 @@ function _system_rebuild_theme_data() {
 
     if (!empty($themes[$key]->info['base theme'])) {
       $sub_themes[] = $key;
+      // Add the defined base theme as dependency to the theme, in order to
+      // re-use the dependency graph resolution of _module_build_dependencies().
+      $themes[$key]->info['dependencies'][] = $themes[$key]->info['base theme'];
     }
     if ($themes[$key]->info['engine'] == 'theme') {
       $filename = dirname($themes[$key]->uri) . '/' . $themes[$key]->name . '.theme';
@@ -2999,30 +3049,36 @@ function _system_rebuild_theme_data() {
  *   Array of all available themes and their data.
  */
 function system_rebuild_theme_data() {
-  $themes = _system_rebuild_theme_data();
-  ksort($themes);
-  // @todo This function has no business in determining/setting the status of
-  //   a theme, but various other functions expect it to return themes with a
-  //   $status property. system_list() stores the return value of this function
-  //   in state, and ensures to set/override the $status property for each theme
-  //   based on the current config. Remove this code when themes have a proper
-  //   installation status.
-  // @see http://drupal.org/node/1067408
-  $enabled_themes = (array) config('system.theme')->get('enabled');
-  $files = array();
-  foreach ($themes as $name => $theme) {
-    $theme->status = (int) isset($enabled_themes[$name]);
-    $files[$name] = $theme->filename;
-  }
-  // Replace last known theme data state.
-  // @todo Obsolete with proper installation status for themes.
-  state()->set('system.theme.data', $themes);
-
-  // Store filenames to allow system_list() and drupal_get_filename() to
-  // retrieve them without having to rebuild or scan the filesystem.
-  state()->set('system.theme.files', $files);
+  $cache = &drupal_static(__FUNCTION__);
+  // Only rebuild once per request. $themes and $cache cannot be combined into
+  // one variable, because $cache is reset by reference from system_list_reset()
+  // during a rebuild.
+  if (!isset($cache)) {
+    $themes = _system_rebuild_theme_data();
+    $files = array();
+    ksort($themes);
+    // Add name, status, and schema version.
+    $enabled_themes = (array) config('system.theme')->get('enabled');
+    $disabled_themes = (array) config('system.theme.disabled')->get();
+    $all_themes = $enabled_themes + $disabled_themes;
+    foreach ($themes as $name => $theme) {
+      $theme->name = $name;
+      $theme->weight = isset($all_themes[$name]) ? $all_themes[$name] : 0;
+      $theme->status = (int) isset($enabled_themes[$name]);
+      $theme->schema_version = SCHEMA_UNINSTALLED;
+      $files[$name] = $theme->filename;
+
+      // Replace last known theme data state.
+      state()->set('system.theme.data.' . $name, $theme);
+    }
+    $themes = _module_build_dependencies($themes);
+    $cache = $themes;
 
-  return $themes;
+    // Store filenames to allow system_list() and drupal_get_filename() to
+    // retrieve them without having to rebuild or scan the filesystem.
+    state()->set('system.theme.files', $files);
+  }
+  return $cache;
 }
 
 /**
@@ -3080,25 +3136,26 @@ function _system_default_theme_features() {
 /**
  * Get a list of available regions from a specified theme.
  *
- * @param $theme_key
- *   The name of a theme.
+ * @param $theme
+ *   A theme object, or the name of a theme.
  * @param $show
  *   Possible values: REGIONS_ALL or REGIONS_VISIBLE. Visible excludes hidden
  *   regions.
  * @return
  *   An array of regions in the form $region['name'] = 'description'.
  */
-function system_region_list($theme_key, $show = REGIONS_ALL) {
-  $themes = list_themes();
-  if (!isset($themes[$theme_key])) {
-    return array();
+function system_region_list($theme, $show = REGIONS_ALL) {
+  if (!is_object($theme)) {
+    $themes = list_themes();
+    if (!isset($themes[$theme])) {
+      return array();
+    }
+    $theme = $themes[$theme];
   }
-
   $list = array();
-  $info = $themes[$theme_key]->info;
   // If requested, suppress hidden regions. See block_admin_display_form().
-  foreach ($info['regions'] as $name => $label) {
-    if ($show == REGIONS_ALL || !isset($info['regions_hidden']) || !in_array($name, $info['regions_hidden'])) {
+  foreach ($theme->info['regions'] as $name => $label) {
+    if ($show == REGIONS_ALL || !isset($theme->info['regions_hidden']) || !in_array($name, $theme->info['regions_hidden'])) {
       $list[$name] = t($label);
     }
   }
diff --git a/core/modules/taxonomy/lib/Drupal/taxonomy/Tests/ThemeTest.php b/core/modules/taxonomy/lib/Drupal/taxonomy/Tests/ThemeTest.php
index b226970..6ab0e74 100644
--- a/core/modules/taxonomy/lib/Drupal/taxonomy/Tests/ThemeTest.php
+++ b/core/modules/taxonomy/lib/Drupal/taxonomy/Tests/ThemeTest.php
@@ -25,8 +25,8 @@ function setUp() {
 
     // Make sure we are using distinct default and administrative themes for
     // the duration of these tests.
+    theme_enable(array('bartik', 'seven'));
     variable_set('theme_default', 'bartik');
-    theme_enable(array('seven'));
     variable_set('admin_theme', 'seven');
 
     // Create and log in as a user who has permission to add and edit taxonomy