diff --git a/core/modules/system/src/Tests/Theme/StableThemeTest.php b/core/modules/system/src/Tests/Theme/StableThemeTest.php
index 996d8b4..06866b5 100644
--- a/core/modules/system/src/Tests/Theme/StableThemeTest.php
+++ b/core/modules/system/src/Tests/Theme/StableThemeTest.php
@@ -10,7 +10,7 @@
 use Drupal\simpletest\KernelTestBase;
 
 /**
- * Tests the behavior of Stable theme.
+ * Tests the behavior of the Stable theme.
  *
  * @group Theme
  */
@@ -26,14 +26,14 @@ class StableThemeTest extends KernelTestBase {
   /**
    * The theme handler.
    *
-   * @var \Drupal\Core\Extension\ThemeHandlerInterface.
+   * @var \Drupal\Core\Extension\ThemeHandlerInterface
    */
   protected $themeHandler;
 
   /**
    * The theme manager.
    *
-   * @var \Drupal\Core\Theme\ThemeManagerInterface.
+   * @var \Drupal\Core\Theme\ThemeManagerInterface
    */
   protected $themeManager;
 
@@ -48,7 +48,7 @@ public function setUp() {
   }
 
   /**
-   * Ensure Stable is used by default when no base theme has been defined.
+   * Ensures Stable is used by default when no base theme has been defined.
    */
   public function testStableIsDefault() {
     $this->themeHandler->install(['test_stable']);
@@ -57,11 +57,11 @@ public function testStableIsDefault() {
     /** @var \Drupal\Core\Theme\ActiveTheme $base_theme */
     $base_themes = $theme->getBaseThemes();
     $base_theme = reset($base_themes);
-    $this->assertTrue($base_theme->getName() == 'stable', "Stable theme has been set on if theme haven't decided to opt-out.");
+    $this->assertTrue($base_theme->getName() == 'stable', "Stable theme is the base theme if a theme hasn't decided to opt out.");
   }
 
   /**
-   * Ensure it is possible to disable Stable by setting the base theme to false.
+   * Tests opting out of Stable by setting the base theme to false.
    */
   public function testWildWest() {
     $this->themeHandler->install(['test_wild_west']);
diff --git a/core/modules/system/src/Tests/Update/StableBaseThemeUpdateTest.php b/core/modules/system/src/Tests/Update/StableBaseThemeUpdateTest.php
index 6621240..f782243 100644
--- a/core/modules/system/src/Tests/Update/StableBaseThemeUpdateTest.php
+++ b/core/modules/system/src/Tests/Update/StableBaseThemeUpdateTest.php
@@ -8,7 +8,7 @@
 namespace Drupal\system\Tests\Update;
 
 /**
- * Tests the upgrade path for the Stable base theme.
+ * Tests the upgrade path for introducing the Stable base theme.
  *
  * @see https://www.drupal.org/node/2575421
  *
@@ -17,6 +17,13 @@
 class StableBaseThemeUpdateTest extends UpdatePathTestBase {
 
   /**
+   * The theme handler.
+   *
+   * @var \Drupal\Core\Extension\ThemeHandlerInterface
+   */
+  protected $themeHandler;
+
+  /**
    * {@inheritdoc}
    */
   protected function setDatabaseDumpFiles() {
@@ -31,25 +38,22 @@ protected function setDatabaseDumpFiles() {
    */
   protected function setUp() {
     parent::setUp();
-    /** @var \Drupal\Core\Extension\ThemeHandlerInterface $theme_handler */
-    $theme_handler = \Drupal::service('theme_handler');
-    $theme_handler->refreshInfo();
+    $this->themeHandler = $this->container->get('theme_handler');
+    $this->themeHandler->refreshInfo();
   }
 
   /**
    * Tests that the Stable base theme is installed if necessary.
    */
   public function testUpdateHookN() {
-    /** @var \Drupal\Core\Extension\ThemeHandlerInterface $theme_handler */
-    $theme_handler = \Drupal::service('theme_handler');
-    $this->assertTrue($theme_handler->themeExists('test_stable'));
-    $this->assertFalse($theme_handler->themeExists('stable'));
+    $this->assertTrue($this->themeHandler->themeExists('test_stable'));
+    $this->assertFalse($this->themeHandler->themeExists('stable'));
 
     $this->runUpdates();
 
     // Refresh the theme handler now that Stable has been installed.
-    $theme_handler->refreshInfo();
-    $this->assertTrue($theme_handler->themeExists('stable'));
+    $this->themeHandler->refreshInfo();
+    $this->assertTrue($this->themeHandler->themeExists('stable'));
   }
 
 }