Update to Drupal 8.2.0. For more information, see https://www.drupal.org/project/drupal/releases/8.2.0

core/tests/Drupal/FunctionalJavascriptTests/Ajax/AjaxThemeTest.php

@@ -0,0 +1,45 @@
+<?php
+
+namespace Drupal\FunctionalJavascriptTests\Ajax;
+
+use Drupal\FunctionalJavascriptTests\JavascriptTestBase;
+
+/**
+ * Tests that AJAX responses use the current theme.
+ *
+ * @group Ajax
+ */
+class AjaxThemeTest extends JavascriptTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public static $modules = ['ajax_test'];
+
+  public function testAjaxWithAdminRoute() {
+    \Drupal::service('theme_installer')->install(['stable', 'seven']);
+    $theme_config = \Drupal::configFactory()->getEditable('system.theme');
+    $theme_config->set('admin', 'seven');
+    $theme_config->set('default', 'stable');
+    $theme_config->save();
+
+    $account = $this->drupalCreateUser(['view the administration theme']);
+    $this->drupalLogin($account);
+
+    // First visit the site directly via the URL. This should render it in the
+    // admin theme.
+    $this->drupalGet('admin/ajax-test/theme');
+    $assert = $this->assertSession();
+    $assert->pageTextContains('Current theme: seven');
+
+    // Now click the modal, which should also use the admin theme.
+    $this->drupalGet('ajax-test/dialog');
+    $assert->pageTextNotContains('Current theme: stable');
+    $this->clickLink('Link 8 (ajax)');
+    $assert->assertWaitOnAjaxRequest();
+
+    $assert->pageTextContains('Current theme: stable');
+    $assert->pageTextNotContains('Current theme: seven');
+  }
+
+}