diff --git a/core/includes/theme.maintenance.inc b/core/includes/theme.maintenance.inc
index 9629ca2..80cc52c 100644
--- a/core/includes/theme.maintenance.inc
+++ b/core/includes/theme.maintenance.inc
@@ -82,6 +82,10 @@ function _drupal_maintenance_theme() {
   $theme = $custom_theme;
 
   // Find all our ancestor themes and put them in an array.
+  // @todo This is just a workaround. Find a better way how to handle themes
+  //   on maintenance pages, see https://www.drupal.org/node/2322619.
+  // This code is basically a duplicate of
+  // \Drupal\Core\Theme\ThemeInitialization::getActiveThemeByName.
   $base_themes = [];
   $ancestor = $theme;
   while ($ancestor && isset($themes[$ancestor]->base_theme)) {
@@ -92,8 +96,6 @@ function _drupal_maintenance_theme() {
       $theme_handler->addTheme($themes[$ancestor]);
     }
   }
-  // @todo This is just a workaround. Find a better way how to handle themes
-  //   on maintenance pages, see https://www.drupal.org/node/2322619.
   \Drupal::theme()->setActiveTheme($theme_init->getActiveTheme($themes[$custom_theme], $base_themes));
   // Prime the theme registry.
   Drupal::service('theme.registry');
diff --git a/core/modules/system/src/Tests/Installer/InstallerThemeTest.php b/core/modules/system/src/Tests/Installer/InstallerThemeTest.php
deleted file mode 100644
index 04ebab9..0000000
--- a/core/modules/system/src/Tests/Installer/InstallerThemeTest.php
+++ /dev/null
@@ -1,45 +0,0 @@
-<?php
-
-/**
- * @file
- * Contains \Drupal\system\Tests\Installer\InstallerThemeTest.
- */
-
-namespace Drupal\system\Tests\Installer;
-
-use Drupal\simpletest\KernelTestBase;
-
-/**
- * Tests themes and base themes are correctly loaded.
- *
- * @group Installer
- */
-class InstallerThemeTest extends KernelTestBase {
-
-  /**
-   * Tests themes and base themes are correctly loaded.
-   *
-   * Regression test for: https://www.drupal.org/node/2614014
-   */
-  protected function testMaintenanceTheme() {
-    $this->settingsSet('maintenance_theme', 'seven');
-    // Get the maintenance theme loaded.
-    drupal_maintenance_theme();
-    // Do we have an active theme?
-    $this->assertTrue(\Drupal::theme()->hasActiveTheme());
-
-    $active_theme = \Drupal::theme()->getActiveTheme();
-    $this->assertEqual($active_theme->getName(), 'seven', 'Active maintenance theme is Seven');
-    $base_themes = $active_theme->getBaseThemes();
-    $base_theme_names = array_keys($base_themes);
-    // Ensure count and order are correct.
-    $this->assertTrue(count($base_themes) === 2);
-    $this->assertEqual($base_theme_names[0], 'classy', 'Classy is the base theme of Seven');
-    $this->assertEqual($base_theme_names[1], 'stable', 'Stable is the parent of Classy');
-    // Ensure Classy has the correct base themes and amount of base themes.
-    $classy_base_themes = $base_themes['classy']->getBaseThemes();
-    $this->assertTrue(count($classy_base_themes) === 1);
-    $this->assertTrue(isset($classy_base_themes['stable']));
-  }
-
-}
diff --git a/core/modules/system/src/Tests/Installer/StandardInstallerTest.php b/core/modules/system/src/Tests/Installer/StandardInstallerTest.php
index 52bd25a..dfe920b 100644
--- a/core/modules/system/src/Tests/Installer/StandardInstallerTest.php
+++ b/core/modules/system/src/Tests/Installer/StandardInstallerTest.php
@@ -44,7 +44,9 @@ protected function setUpSite() {
    * {@inheritdoc}
    */
   protected function curlExec($curl_options, $redirect = FALSE) {
-    // Ensure that we see the classy progress CSS on the page.
+    // Ensure that we see the classy progress CSS on the batch page.
+    // Batch processing happens as part of HTTP redirects, so we can access the
+    // HTML of the batch page.
     if (strpos($curl_options[CURLOPT_URL], '&id=1&op=do_nojs') !== FALSE) {
       $this->assertRaw('themes/classy/css/components/progress.css');
     }
diff --git a/core/tests/Drupal/KernelTests/Core/Theme/MaintenanceThemeTest.php b/core/tests/Drupal/KernelTests/Core/Theme/MaintenanceThemeTest.php
new file mode 100644
index 0000000..06ed04b
--- /dev/null
+++ b/core/tests/Drupal/KernelTests/Core/Theme/MaintenanceThemeTest.php
@@ -0,0 +1,43 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\KernelTests\Core\Theme\MaintenanceThemeTest.
+ */
+
+namespace Drupal\KernelTests\Core\Theme;
+
+use Drupal\KernelTests\KernelTestBase;
+
+/**
+ * Tests themes and base themes are correctly loaded.
+ *
+ * @group Installer
+ */
+class MaintenanceThemeTest extends KernelTestBase {
+
+  /**
+   * Ensures that the maintenance theme initializes theme and their base themes.
+   */
+  public function testMaintenanceTheme() {
+    $this->setSetting('maintenance_theme', 'seven');
+    // Get the maintenance theme loaded.
+    drupal_maintenance_theme();
+
+    // Do we have an active theme?
+    $this->assertTrue(\Drupal::theme()->hasActiveTheme());
+
+    $active_theme = \Drupal::theme()->getActiveTheme();
+    $this->assertEquals('seven', $active_theme->getName());
+
+    $base_themes = $active_theme->getBaseThemes();
+    $base_theme_names = array_keys($base_themes);
+    $this->assertSame(['classy', 'stable'], $base_theme_names);
+
+    // Ensure Classy has the correct base themes and amount of base themes.
+    $classy_base_themes = $base_themes['classy']->getBaseThemes();
+    $classy_base_theme_names = array_keys($classy_base_themes);
+    $this->assertSame(['stable'], $classy_base_theme_names);
+  }
+
+}