--- modules/system/system.module
+++ modules/system/system.module
@@ -2681,6 +2681,7 @@
   else {
     $options = array('path' => $path);
   }
+  $options['attributes'] = array('class' => 'button');
   $cancel = l($no ? $no : t('Cancel'), $options['path'], $options);
 
   drupal_set_title($question, PASS_THROUGH);