Drupal 8.0.0 beta 12. More info: https://www.drupal.org/node/2514176

core/modules/system/src/Theme/BatchNegotiator.php

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+<?php
+
+/**
+ * @file
+ * Contains \Drupal\system\Theme\BatchNegotiator.
+ */
+
+namespace Drupal\system\Theme;
+
+use Drupal\Core\Batch\BatchStorageInterface;
+use Drupal\Core\Routing\RouteMatchInterface;
+use Drupal\Core\Theme\ThemeNegotiatorInterface;
+use Symfony\Component\HttpFoundation\RequestStack;
+
+/**
+ * Sets the active theme for the batch page.
+ */
+class BatchNegotiator implements ThemeNegotiatorInterface {
+
+  /**
+   * The batch storage.
+   *
+   * @var \Drupal\Core\Batch\BatchStorageInterface
+   */
+  protected $batchStorage;
+
+  /**
+   * The request stack.
+   *
+   * @var \Symfony\Component\HttpFoundation\RequestStack
+   */
+  protected $requestStack;
+
+  /**
+   * Constructs a BatchNegotiator.
+   *
+   * @param \Drupal\Core\Batch\BatchStorageInterface $batch_storage
+   *   The batch storage.
+   * @param \Symfony\Component\HttpFoundation\RequestStack $request_stack
+   *   The request stack used to retrieve the current request.
+   */
+  public function __construct(BatchStorageInterface $batch_storage, RequestStack $request_stack) {
+    $this->batchStorage = $batch_storage;
+    $this->requestStack = $request_stack;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function applies(RouteMatchInterface $route_match) {
+    return $route_match->getRouteName() == 'system.batch_page';
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function determineActiveTheme(RouteMatchInterface $route_match) {
+    // Retrieve the current state of the batch.
+    $request = $this->requestStack->getCurrentRequest();
+    $batch = &batch_get();
+    if (!$batch && $request->request->has('id')) {
+      $batch = $this->batchStorage->load($request->request->get('id'));
+    }
+    // Use the same theme as the page that started the batch.
+    if (!empty($batch['theme'])) {
+      return $batch['theme'];
+    }
+  }
+
+}

core/modules/system/src/Theme/DbUpdateNegotiator.php

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+<?php
+
+/**
+ * @file
+ * Contains \Drupal\system\Theme\DbUpdateNegotiator.
+ */
+
+namespace Drupal\system\Theme;
+
+use Drupal\Core\Config\ConfigFactoryInterface;
+use Drupal\Core\Routing\RouteMatchInterface;
+use Drupal\Core\Site\Settings;
+use Drupal\Core\Theme\ThemeNegotiatorInterface;
+
+/**
+ * Sets the active theme for the database update pages.
+ */
+class DbUpdateNegotiator implements ThemeNegotiatorInterface {
+
+  /**
+   * The config factory.
+   *
+   * @var \Drupal\Core\Config\ConfigFactoryInterface
+   */
+  protected $configFactory;
+
+  /**
+   * Constructs a DbUpdateNegotiator.
+   *
+   * @param \Drupal\Core\Config\ConfigFactoryInterface $config_factory
+   *   The config factory.
+   */
+  public function __construct(ConfigFactoryInterface $config_factory) {
+    $this->configFactory = $config_factory;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function applies(RouteMatchInterface $route_match) {
+    return $route_match->getRouteName() == 'system.db_update';
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function determineActiveTheme(RouteMatchInterface $route_match) {
+    $custom_theme = Settings::get('maintenance_theme', 'seven');
+    if (!$custom_theme) {
+      $config = $this->configFactory->get('system.theme');
+      $custom_theme = $config->get('default');
+    }
+
+    return $custom_theme;
+  }
+
+}