diff --git a/includes/theme.inc b/includes/theme.inc
index c211248..c42999b 100644
--- a/includes/theme.inc
+++ b/includes/theme.inc
@@ -279,28 +279,7 @@ function _theme_registry_callback($callback = NULL, array $arguments = array())
  *   The name of the theme engine.
  */
 function _theme_load_registry($theme, $base_theme = NULL, $theme_engine = NULL) {
-  // Check the theme registry cache; if it exists, use it.
-  $cache = cache_get("theme_registry:$theme->name", 'cache');
-  if (isset($cache->data)) {
-    $registry = $cache->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 $registry;
-}
-
-/**
- * Write the theme_registry cache into the database.
- */
-function _theme_save_registry($theme, $registry) {
-  cache_set("theme_registry:$theme->name", $registry);
+  return new ThemeRegistry($theme, $base_theme, $theme_engine);
 }
 
 /**
@@ -563,6 +542,113 @@ function _theme_build_registry($theme, $base_theme, $theme_engine) {
 }
 
 /**
+ * Builds the run-time theme registry.
+ *
+ * This class extends the ArrayObject class 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 loade and used to update the run-time cache.
+ */
+class ThemeRegistry Extends ArrayObject {
+  private $theme;
+  private $base_theme;
+  private $theme_engine;
+  private $cid;
+  private $bin;
+
+  function __construct($theme, $base_theme = NULL, $theme_engine = NULL) {
+    // Make the arguments available to the rest of the class.
+    $this->theme = $theme;
+    $this->base_theme = $base_theme;
+    $this->theme_engine = $theme_engine;
+    $this->cid = 'theme_registry:' . $theme->name;
+    $this->bin = 'cache';
+
+    // Load the run-time theme registry from cache if available.
+    if ($cached = cache_get($this->cid, $this->bin)) {
+      parent::__construct($cached->data);
+    }
+    else {
+      parent::__construct(array());
+    }
+  }
+
+  public function offsetExists($offset) {
+    // The values of the theme registry are always arrays, so ensure that the
+    // value is non-empty.
+    if (!empty($this->$offset)) {
+      return TRUE;
+    }
+    // If the value is isset but empty, this is a cached miss for a theme key.
+    else {
+      if (isset($this->$offset)) {
+        return FALSE;
+      }
+      else {
+        // If the key is not available, load it from the complete theme
+        // registry, then re-run this method.
+        $this->resolveCacheMiss($offset);
+        if (!empty($this->$offset)) {
+          return TRUE;
+        }
+        else if (isset($this->$offset)) {
+          return FALSE;
+        }
+      }
+    }
+  }
+
+  public function offsetGet($offset) {
+    if (!empty($this->$offset)) {
+      return $this->$offset;
+    }
+    else {
+      if (isset($this->$offset)) {
+        return NULL;
+      }
+      else {
+        $this->resolveCacheMiss($offset);
+        return $this->$offset;
+      }
+    }
+  }
+
+  /**
+   * Resolve cache misses from the run-time registry.
+   *
+   * @param $offset
+   *   The array key requested.
+   */
+  public function resolveCacheMiss($offset) {
+    $this->cacheMiss = TRUE;
+    // Load the full theme registry.
+    if ($cached = cache_get('theme_registry:complete:' . $this->theme->name)) {
+      $complete_registry = $cached->data;
+    }
+    else {
+      $complete_registry = _theme_build_registry($this->theme, $this->base_theme, $this->theme_engine);
+      // Only persist this registry if all modules are loaded. This assures a
+      // complete set of theme hooks.
+      if (module_load_all(NULL)) {
+        cache_set('theme_registry:complete:' . $this->theme->name, $complete_registry);
+      }
+    }
+    if (isset($complete_registry[$offset])) {
+      $this->$offset = $complete_registry[$offset];
+    }
+    else {
+      $this->$offset = FALSE;
+    }
+  }
+
+  public function __destruct() {
+    if (!empty($this->cacheMiss) && module_load_all(NULL)) {
+      cache_set($this->cid, $this->getarrayCopy(), $this->bin);
+    }
+  }
+}
+
+/**
  * Return a list of all currently available themes.
  *
  * Retrieved from the database, if available and the site is not in maintenance