--- /dev/null
+<?php
+
+namespace Drupal\user\Theme;
+
+use Drupal\Core\Config\ConfigFactoryInterface;
+use Drupal\Core\Entity\EntityManagerInterface;
+use Drupal\Core\Routing\AdminContext;
+use Drupal\Core\Routing\RouteMatchInterface;
+use Drupal\Core\Session\AccountInterface;
+use Drupal\Core\Theme\ThemeNegotiatorInterface;
+
+/**
+ * Sets the active theme on admin pages.
+ */
+class AdminNegotiator implements ThemeNegotiatorInterface {
+
+ /**
+ * The current user.
+ *
+ * @var \Drupal\Core\Session\AccountInterface
+ */
+ protected $user;
+
+ /**
+ * The config factory.
+ *
+ * @var \Drupal\Core\Config\ConfigFactoryInterface
+ */
+ protected $configFactory;
+
+ /**
+ * The entity manager.
+ *
+ * @var \Drupal\Core\Entity\EntityManagerInterface
+ */
+ protected $entityManager;
+
+ /**
+ * The route admin context to determine whether a route is an admin one.
+ *
+ * @var \Drupal\Core\Routing\AdminContext
+ */
+ protected $adminContext;
+
+ /**
+ * Creates a new AdminNegotiator instance.
+ *
+ * @param \Drupal\Core\Session\AccountInterface $user
+ * The current user.
+ * @param \Drupal\Core\Config\ConfigFactoryInterface $config_factory
+ * The config factory.
+ * @param \Drupal\Core\Entity\EntityManagerInterface $entity_manager
+ * The entity manager.
+ * @param \Drupal\Core\Routing\AdminContext $admin_context
+ * The route admin context to determine whether the route is an admin one.
+ */
+ public function __construct(AccountInterface $user, ConfigFactoryInterface $config_factory, EntityManagerInterface $entity_manager, AdminContext $admin_context) {
+ $this->user = $user;
+ $this->configFactory = $config_factory;
+ $this->entityManager = $entity_manager;
+ $this->adminContext = $admin_context;
+ }
+
+ /**
+ * {@inheritdoc}
+ */
+ public function applies(RouteMatchInterface $route_match) {
+ return ($this->entityManager->hasHandler('user_role', 'storage') && $this->user->hasPermission('view the administration theme') && $this->adminContext->isAdminRoute($route_match->getRouteObject()));
+ }
+
+ /**
+ * {@inheritdoc}
+ */
+ public function determineActiveTheme(RouteMatchInterface $route_match) {
+ return $this->configFactory->get('system.theme')->get('admin');
+ }
+
+}