.../modules/help_topics/tests/src/Functional/HelpTopicSearchTest.php | 5 +++++
 .../Drupal/FunctionalJavascriptTests/Ajax/MessageCommandTest.php     | 5 +++++
 2 files changed, 10 insertions(+)

diff --git a/core/modules/help_topics/tests/src/Functional/HelpTopicSearchTest.php b/core/modules/help_topics/tests/src/Functional/HelpTopicSearchTest.php
index a886167700..2f6765c151 100644
--- a/core/modules/help_topics/tests/src/Functional/HelpTopicSearchTest.php
+++ b/core/modules/help_topics/tests/src/Functional/HelpTopicSearchTest.php
@@ -23,6 +23,11 @@ class HelpTopicSearchTest extends HelpTopicTranslatedTestBase {
     'language',
   ];
 
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'classy';
+
   /**
    * {@inheritdoc}
    */
diff --git a/core/tests/Drupal/FunctionalJavascriptTests/Ajax/MessageCommandTest.php b/core/tests/Drupal/FunctionalJavascriptTests/Ajax/MessageCommandTest.php
index 22cbbab76e..04b824c574 100644
--- a/core/tests/Drupal/FunctionalJavascriptTests/Ajax/MessageCommandTest.php
+++ b/core/tests/Drupal/FunctionalJavascriptTests/Ajax/MessageCommandTest.php
@@ -16,6 +16,11 @@ class MessageCommandTest extends WebDriverTestBase {
    */
   protected static $modules = ['ajax_test'];
 
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
   /**
    * Test AJAX MessageCommand use in a form.
    */