reverted:
--- b/core/modules/system/system.routing.yml
+++ a/core/modules/system/system.routing.yml
@@ -143,10 +143,3 @@
     _content: 'Drupal\system\Controller\AdminController::index'
   requirements:
     _permission: 'access administration pages'
-
-system_theme_default:
-  pattern: '/admin/appearance/default'
-  defaults:
-    _controller: 'Drupal\system\Controller\ThemeController::defaultTheme'
-  requirements:
-    _permission: 'administer themes'