diff --git a/includes/theme.inc b/includes/theme.inc
index 6c2b640..bb031f1 100644
--- a/includes/theme.inc
+++ b/includes/theme.inc
@@ -237,18 +237,33 @@ function _drupal_theme_initialize($theme, $base_theme = array(), $registry_callb
 /**
  * Get the theme registry.
  *
+ * @param $complete
+ *   Optional boolean to indicate whether to return the complete theme registry
+ *   array or an instance of the 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 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
- *   The theme registry array if it has been stored in memory, NULL otherwise.
+ *   The complete theme registry array, or an instance of the ThemeRegistry
+ *   class.
  */
-function theme_get_registry() {
-  static $theme_registry = NULL;
+function theme_get_registry($complete = TRUE) {
+  static $theme_registry = array();
+  $key = (int) $complete;
 
-  if (!isset($theme_registry)) {
+  if (!isset($theme_registry[$key])) {
     list($callback, $arguments) = _theme_registry_callback();
-    $theme_registry = call_user_func_array($callback, $arguments);
+    if (!$complete) {
+      $arguments[] = FALSE;
+    }
+    $theme_registry[$key] = call_user_func_array($callback, $arguments);
   }
 
-  return $theme_registry;
+  return $theme_registry[$key];
 }
 
 /**
@@ -268,7 +283,7 @@ function _theme_registry_callback($callback = NULL, array $arguments = array())
 }
 
 /**
- * Get the theme_registry cache from the database; if it doesn't exist, build it.
+ * Get the theme_registry cache; if it doesn't exist, build it.
  *
  * @param $theme
  *   The loaded $theme object as returned by list_themes().
@@ -277,23 +292,34 @@ function _theme_registry_callback($callback = NULL, array $arguments = array())
  *   oldest first order.
  * @param theme_engine
  *   The name of the theme engine.
+ * @param $complete
+ *   Whether to load the complete theme registry or an instance of the
+ *   ThemeRegistry class.
+ *
+ * @return
+ *   The theme registry array, or an instance of the ThemeRegistry class.
  */
-function _theme_load_registry($theme, $base_theme = NULL, $theme_engine = NULL) {
-  // Check the theme registry cache; if it exists, use it.
-  $cache = cache()->get("theme_registry:$theme->name");
-  if (isset($cache->data)) {
-    $registry = $cache->data;
+function _theme_load_registry($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 {
-    // 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 new ThemeRegistry('theme_registry:runtime:' . $theme->name, 'cache');
   }
-  return $registry;
 }
 
 /**
@@ -313,6 +339,104 @@ function drupal_theme_rebuild() {
 }
 
 /**
+ * Builds the run-time theme registry.
+ *
+ * Extends DrupalCacheArray to allow the theme registry to be accessed as a
+ * complete registry, while internally caching only the parts of the registry
+ * that are actually in use on the site. On cache misses the complete
+ * theme registry is loaded and used to update the run-time cache.
+ */
+class ThemeRegistry Extends DrupalCacheArray {
+
+  /**
+   * Whether the partial registry can be persisted to the cache.
+   *
+   * This is only allowed if all modules and the request method is GET. theme()
+   * should be very rarely called on POST requests and this avoids polluting
+   * the runtime cache.
+   */
+  private $persistable;
+
+  /**
+   * The complete theme registry array.
+   */
+  private $completeRegistry;
+
+  function __construct($cid, $bin) {
+    $this->cid = $cid;
+    $this->bin = $bin;
+    $this->persistable = module_load_all(NULL) && $_SERVER['REQUEST_METHOD'] == 'GET';
+
+    $data = array();
+    if ($this->persistable && $cached = cache($this->bin)->get($this->cid)) {
+      $data = $cached->data;
+    }
+    else {
+      $complete_registry = theme_get_registry();
+      if ($this->persistable) {
+        // If there is no runtime cache stored, fetch the full theme registry,
+        // but then initialize each value to NULL. This allows
+        // offsetExists() to function correctly on non-registered theme hooks
+        // without triggering a call to resolveCacheMiss().
+        $data = array_fill_keys(array_keys($complete_registry), NULL);
+        $this->set($this->cid, $data, $this->bin);
+        $this->completeRegistry = $complete_registry;
+      }
+      else {
+        $data = $complete_registry;
+      }
+    }
+    $this->storage = $data;
+  }
+
+  public function offsetExists($offset) {
+    // Since the theme registry allows for theme hooks to be requested that
+    // are not registered, just check the existence of the key in the registry.
+    // Use array_key_exists() here since a NULL value indicates that the theme
+    // hook exists but has not yet been requested.
+    return array_key_exists($offset, $this->storage);
+  }
+
+  public function offsetGet($offset) {
+    // If the offset is set but empty, it is a registered theme hook that has
+    // not yet been requested. Offsets that do not exist at all were not
+    // registered in hook_theme().
+    if (isset($this->storage[$offset])) {
+      return $this->storage[$offset];
+    }
+    elseif (array_key_exists($offset, $this->storage)) {
+      return $this->resolveCacheMiss($offset);
+    }
+  }
+
+  public function resolveCacheMiss($offset) {
+    if (!isset($this->completeRegistry)) {
+      $this->completeRegistry = theme_get_registry();
+    }
+    $this->storage[$offset] = $this->completeRegistry[$offset];
+    if ($this->persistable) {
+      $this->persist($offset);
+    }
+    return $this->storage[$offset];
+  }
+
+  public function set($cid, $data, $bin, $lock = TRUE) {
+    $lock_name = $cid . ':' . $bin;
+    if (!$lock || lock_acquire($lock_name)) {
+      if ($cached = cache($bin)->get($cid)) {
+        // Use array merge instead of union so that filled in values in $data
+        // overwrite empty values in the current cache.
+        $data = array_merge($cached->data, $data);
+      }
+      cache($bin)->set($cid, $data);
+      if ($lock) {
+        lock_release($lock_name);
+      }
+    }
+  }
+}
+
+/**
  * Process a single implementation of hook_theme().
  *
  * @param $cache
@@ -760,7 +884,7 @@ function theme($hook, $variables = array()) {
 
   if (!isset($hooks)) {
     drupal_theme_initialize();
-    $hooks = theme_get_registry();
+    $hooks = theme_get_registry(FALSE);
   }
 
   // If an array of hook candidates were passed, use the first one that has an
diff --git a/modules/contextual/contextual.module b/modules/contextual/contextual.module
index 0d6b625..2716ba2 100644
--- a/modules/contextual/contextual.module
+++ b/modules/contextual/contextual.module
@@ -82,16 +82,12 @@ function contextual_element_info() {
  * @see contextual_pre_render_links()
  */
 function contextual_preprocess(&$variables, $hook) {
-  static $hooks;
-
   // Nothing to do here if the user is not permitted to access contextual links.
   if (!user_access('access contextual links')) {
     return;
   }
 
-  if (!isset($hooks)) {
-    $hooks = theme_get_registry();
-  }
+  $hooks = theme_get_registry(FALSE);
 
   // Determine the primary theme function argument.
   if (!empty($hooks[$hook]['variables'])) {
diff --git a/modules/simpletest/tests/theme.test b/modules/simpletest/tests/theme.test
index f1e1bd5..2529e8d 100644
--- a/modules/simpletest/tests/theme.test
+++ b/modules/simpletest/tests/theme.test
@@ -194,7 +194,7 @@ class ThemeItemListUnitTest extends DrupalWebTestCase {
 /**
  * Unit tests for theme_links().
  */
-class ThemeLinksUnitTest extends DrupalUnitTestCase {
+class ThemeLinksTest extends DrupalWebTestCase {
   public static function getInfo() {
     return array(
       'name' => 'Links',