--- modules/system/system.admin.inc	2010-08-27 13:30:03.000000000 +0300
+++ modules/system/system.admin.inc	2010-08-27 13:20:14.000000000 +0300
@@ -270,6 +270,15 @@ function system_themes_page() {
             'query' => $query,
             'attributes' => array('title' => t('Set !theme as default theme', array('!theme' => $theme->info['name']))),
           );
+        } else {
+          $theme->operations[] = array(
+            'title' => t('Disable'),
+            'attributes' => array('title' => t('Cannot disable default theme'))
+          );
+          $theme->operations[] = array(
+            'title' => t('Set default'),
+            'attributes' => array('title' => t('Already default'))
+          );
         }
       }
       else {