diff --git includes/theme.inc includes/theme.inc
index baceb73..0c5df7c 100644
--- includes/theme.inc
+++ includes/theme.inc
@@ -67,14 +67,34 @@ function _drupal_theme_access($theme) {
 
 /**
  * Initialize the theme system by loading the theme.
+ * Use $force_theme to load a specific theme
  */
-function drupal_theme_initialize() {
-  global $theme, $user, $theme_key;
+function drupal_theme_initialize($force_theme = '') {
+  global $theme, $user, $theme_key;
+  if (defined('MAINTENANCE_MODE')) {
+    $drupal_theme_initialize_rebuild = &drupal_static('theme_switch_drupal_theme_initialize_rebuild', FALSE);
+    $drupal_theme_initialize_rebuild = TRUE;
+  }
+  else {
+    $drupal_theme_initialize_rebuild = &drupal_static('theme_switch_drupal_theme_initialize_rebuild', FALSE);
+  }
 
   // If $theme is already set, assume the others are set, too, and do nothing
-  if (isset($theme)) {
+  //unless force_theme is set, or static caches are cleared
+  if (isset($theme) && $force_theme == '' && $drupal_theme_initialize_rebuild) {
     return;
   }
+  elseif ($force_theme != '') {
+    drupal_static_reset('theme_switch_theme');
+    drupal_static_reset('theme_switch_theme_get_registry');
+    drupal_static_reset('theme_switch__theme_registry_callback');
+    drupal_static_reset('drupal_add_css');
+    drupal_static_reset('drupal_add_js');
+    drupal_theme_rebuild();
+  }
+  $drupal_theme_initialize_rebuild = TRUE;
 
   drupal_bootstrap(DRUPAL_BOOTSTRAP_DATABASE);
   $themes = list_themes();
@@ -86,7 +106,7 @@ function drupal_theme_initialize() {
   // Allow modules to override the theme. Validation has already been performed
   // inside menu_get_custom_theme(), so we do not need to check it again here.
   $custom_theme = menu_get_custom_theme();
-  $theme = !empty($custom_theme) ? $custom_theme : $theme;
+  $theme = !empty($force_theme) ? $force_theme : (!empty($custom_theme) ? $custom_theme : $theme);
 
   // Store the identifier for retrieving theme settings with.
   $theme_key = $theme;
@@ -242,11 +262,13 @@ function _drupal_theme_initialize($theme, $base_theme = array(), $registry_callb
  *   The theme registry array if it has been stored in memory, NULL otherwise.
  */
 function theme_get_registry() {
-  static $theme_registry = NULL;
+  $theme_registry = &drupal_static('theme_switch_theme_get_registry');
 
   if (!isset($theme_registry)) {
     list($callback, $arguments) = _theme_registry_callback();
-    $theme_registry = call_user_func_array($callback, $arguments);
+    if (isset($callback)) {
+      $theme_registry = call_user_func_array($callback, $arguments);
+    }
   }
 
   return $theme_registry;
@@ -261,10 +283,12 @@ function theme_get_registry() {
  *   The arguments to pass to the function.
  */
 function _theme_registry_callback($callback = NULL, array $arguments = array()) {
-  static $stored;
+  $stored = &drupal_static('theme_switch__theme_registry_callback', NULL);
   if (isset($callback)) {
     $stored = array($callback, $arguments);
   }
   return $stored;
 }
 
@@ -736,7 +760,12 @@ function list_themes($refresh = FALSE) {
  *   An HTML string representing the themed output.
  */
 function theme($hook, $variables = array()) {
-  static $hooks = NULL;
+ // Use the advanced drupal_static() pattern, since this is called very often.
+  static $drupal_static_fast;
+  if (!isset($drupal_static_fast)) {
+    $drupal_static_fast['theme'] = &drupal_static('theme_switch_theme');
+  }
+  $hooks = &$drupal_static_fast['theme'];
 
   // 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
diff --git modules/simpletest/tests/theme.test modules/simpletest/tests/theme.test
index 9e2f387..82bf63b 100644
--- modules/simpletest/tests/theme.test
+++ modules/simpletest/tests/theme.test
@@ -188,6 +188,44 @@ class ThemeItemListUnitTest extends DrupalWebTestCase {
 }
 
 /**
+ * Unit tests for drupal_theme_initialize().
+ */
+class ThemeSwitchUnitTest extends DrupalWebTestCase {
+  public static function getInfo() {
+    return array(
+      'name' => 'Theme Switch',
+      'description' => 'Test the drupal_theme_initialize($force_theme) function to change the theme.',
+      'group' => 'Theme',
+    );
+  }
+
+  /**
+   * Test custom theme rendering.
+   */
+  function testThemeSwitch() {
+    drupal_theme_initialize('seven');
+    $node1 = $this->drupalCreateNode();
+    $output1 = node_view($node1);
+    $output1 = drupal_render_page($output1);
+    $this->assertTheme('seven', 'bartik', path_to_theme(), $output1);
+    drupal_theme_initialize('bartik');
+    $node2 = $this->drupalCreateNode();
+    $output2 = node_view($node2);
+    $output2 = drupal_render_page($output2);
+    $this->assertTheme('bartik', 'seven', path_to_theme(), $output2);
+
+
+  }
+
+  function assertTheme($name, $previous_theme, $path, $output, $message = '', $group = 'Other') {
+    return $this->assert((strpos($path, $name) && strpos($output, $path) && !strpos($output, $previous_theme)),
+                          $message ? $message : t('Switching to theme \'@name\'', array('@name' => $name)),
+                          $group);
+
+  }
+}
+
+/**
  * Unit tests for theme_links().
  */
 class ThemeLinksUnitTest extends DrupalUnitTestCase {