Index: modules/system/system.admin.inc
===================================================================
RCS file: /cvs/drupal/drupal/modules/system/system.admin.inc,v
retrieving revision 1.326
diff -u -p -r1.326 system.admin.inc
--- modules/system/system.admin.inc	23 Nov 2010 02:54:13 -0000	1.326
+++ modules/system/system.admin.inc	26 Dec 2010 04:41:58 -0000
@@ -346,10 +346,18 @@ function system_theme_default() {
     if (!empty($themes[$theme])) {
       // Enable the theme if it is currently disabled.
       if (empty($themes[$theme]->status)) {
-       theme_enable(array($theme));
+        theme_enable(array($theme));
       }
       // Set the default theme.
       variable_set('theme_default', $theme);
+
+      // Rebuild the menu. This duplicates the menu_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_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);
Index: modules/system/system.test
===================================================================
RCS file: /cvs/drupal/drupal/modules/system/system.test,v
retrieving revision 1.153
diff -u -p -r1.153 system.test
--- modules/system/system.test	1 Dec 2010 00:23:36 -0000	1.153
+++ modules/system/system.test	26 Dec 2010 04:42:12 -0000
@@ -1355,7 +1355,7 @@ class SystemThemeFunctionalTest extends 
   function setUp() {
     parent::setUp();
 
-    $this->admin_user = $this->drupalCreateUser(array('access administration pages', 'view the administration theme', 'administer themes', 'bypass node access'));
+    $this->admin_user = $this->drupalCreateUser(array('access administration pages', 'view the administration theme', 'administer themes', 'bypass node access', 'administer blocks'));
     $this->drupalLogin($this->admin_user);
     $this->node = $this->drupalCreateNode();
   }
@@ -1439,6 +1439,26 @@ class SystemThemeFunctionalTest extends 
     $this->drupalGet('node/add');
     $this->assertRaw('themes/bartik', t('Site default theme used on the add content page.'));
   }
+
+  /**
+   * Test switching the default theme.
+   */
+  function testSwitchDefaultTheme() {
+    // Enable "stark" and set it as the default theme.
+    theme_enable(array('stark'));
+    $this->drupalGet('admin/appearance');
+    $this->clickLink(t('Set default'), 1);
+    $this->assertTrue(variable_get('theme_default', '') == 'stark', t('Site default theme switched successfully.'));
+
+    // Test the default theme on the secondary links (blocks admin page).
+    $this->drupalGet('admin/structure/block');
+    $this->assertText('Stark(' . t('active tab') . ')', t('Default local task on blocks admin page is the default theme.'));
+    // Switch back to Bartik and test again to test that the menu cache is cleared.
+    $this->drupalGet('admin/appearance');
+    $this->clickLink(t('Set default'), 0);
+    $this->drupalGet('admin/structure/block');
+    $this->assertText('Bartik(' . t('active tab') . ')', t('Default local task on blocks admin page has changed.'));
+  }
 }