diff -u b/core/lib/Drupal/Core/Extension/ThemeHandler.php b/core/lib/Drupal/Core/Extension/ThemeHandler.php
--- b/core/lib/Drupal/Core/Extension/ThemeHandler.php
+++ b/core/lib/Drupal/Core/Extension/ThemeHandler.php
@@ -388,7 +388,8 @@
         return array();
       }
     }
-    return $base_themes;
+    // The "root" theme is expected to be returned first.
+    return array_reverse($base_themes);
   }
 
   /**