reverted:
--- b/core/modules/system/system.module
+++ a/core/modules/system/system.module
@@ -22,7 +22,6 @@
 use Drupal\Core\Extension\ModuleHandler;
 use Drupal\Core\Url;
 use Drupal\Core\Block\BlockPluginInterface;
-use Drupal\Core\Theme\MissingThemeDependencyException;
 use Drupal\user\UserInterface;
 use Symfony\Component\HttpFoundation\RedirectResponse;
 use GuzzleHttp\Exception\RequestException;
@@ -524,25 +523,8 @@
     $page['#attached']['library'][] = 'system/admin';
   }
 
-  // Band-aid.
-  /** @var \Drupal\Core\Theme\ThemeManagerInterface $theme_manager */
-  $theme_manager = \Drupal::theme();
-  /** @var \Drupal\Core\Extension\ThemeHandlerInterface $theme_handler */
-  $theme_handler = \Drupal::service('theme_handler');
-  $get_active_theme = function () use ($theme_manager, $theme_handler, &$get_active_theme) {
-    try {
-      $active_theme = $theme_manager->getActiveTheme();
-      return $active_theme;
-    }
-    catch (MissingThemeDependencyException $e) {
-      $theme_handler->install([$e->theme]);
-      $themes = $theme_handler->rebuildThemeData();
-      return $get_active_theme();
-    }
-  };
-
   // Attach libraries used by this theme.
+  $active_theme = \Drupal::theme()->getActiveTheme();
-  $active_theme = $get_active_theme();
   foreach ($active_theme->getLibraries() as $library) {
     $page['#attached']['library'][] = $library;
   }