#3288547 Fix Phpunit tests

tests/src/Functional/MenuDisallowExternalLinksTest.php

@@ -16,7 +16,7 @@ class MenuDisallowExternalLinksTest extends BrowserTestBase {
   /**
    * {@inheritdoc}
    */
-  protected $defaultTheme = 'stable';
+  protected $defaultTheme = 'stark';
 
   /**
    * {@inheritdoc}