only in patch2:
unchanged:
--- /dev/null
+++ b/core/modules/user/lib/Drupal/user/Theme/UserNegotiator.php
@@ -0,0 +1,61 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\user\Theme\UserNegotiator.
+ */
+
+namespace Drupal\user\Theme;
+
+use Drupal\Core\Entity\EntityManager;
+use Drupal\Core\Session\AccountInterface;
+use Drupal\Core\Theme\ActiveTheme;
+use Drupal\Core\Theme\ThemeNegotiatorInterface;
+use Symfony\Component\HttpFoundation\Request;
+
+/**
+ * Defines the theme negotiator service for theme configured per user.
+ */
+class UserNegotiator implements  ThemeNegotiatorInterface {
+
+  /**
+   * The user storage controller.
+   *
+   * @var \Drupal\user\UserStorageControllerInterface
+   */
+  protected $userStorageController;
+
+  /**
+   * The current user.
+   *
+   * @var \Drupal\Core\Session\AccountInterface
+   */
+  protected $currentUser;
+
+  /**
+   * Constructs a UserNegotiator object.
+   *
+   * @param \Drupal\Core\Entity\EntityManager $entity_manager
+   *   The entity manager
+   * @param \Drupal\Core\Session\AccountInterface $current_user
+   *   The current user.
+   */
+  public function __construct(EntityManager $entity_manager, AccountInterface $current_user) {
+    $this->userStorageController = $entity_manager->getStorageController('user');
+    $this->currentUser = $current_user;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function determineActiveTheme(Request $request) {
+    if ($user = $this->userStorageController->load($this->currentUser->id())) {;
+      // Only select the user selected theme if it is available in the
+      // list of themes that can be accessed.
+      if (!empty($user->theme) && drupal_theme_access($user->theme)) {
+        return $user->theme;
+      }
+    }
+  }
+
+}