diff --git a/core/lib/Drupal/Core/Extension/ThemeHandler.php b/core/lib/Drupal/Core/Extension/ThemeHandler.php
index d5b3317..ce4f88b 100644
--- a/core/lib/Drupal/Core/Extension/ThemeHandler.php
+++ b/core/lib/Drupal/Core/Extension/ThemeHandler.php
@@ -164,15 +164,7 @@ public function disable(array $theme_list) {
     $admin_theme = \Drupal::config('system.theme')->get('admin');
     $required_themes = array($default_theme, $admin_theme);
 
-    foreach ($required_themes as $theme) {
-      $pos = array_search($theme, $theme_list);
-      if ($pos !== FALSE) {
-        unset($theme_list[$pos]);
-        if (empty($theme_list)) {
-          return;
-        }
-      }
-    }
+    $theme_list = array_diff($theme_list, $required_themes);
 
     $this->clearCssCache();