Index: modules/system/system.admin.inc
===================================================================
RCS file: /cvs/drupal/drupal/modules/system/system.admin.inc,v
retrieving revision 1.298
diff -u -p -r1.298 system.admin.inc
--- modules/system/system.admin.inc	22 Aug 2010 12:55:04 -0000	1.298
+++ modules/system/system.admin.inc	23 Aug 2010 13:16:03 -0000
@@ -435,6 +435,7 @@ function system_theme_default() {
       else {
         drupal_set_message(t('%theme is now the default theme.', array('%theme' => $themes[$theme]->info['name'])));
       }
+      menu_rebuild();
     }
     else {
       drupal_set_message(t('The %theme theme was not found.', array('%theme' => $theme)), 'error');