diff --git a/core/modules/big_pipe/tests/modules/big_pipe_test/src/BigPipePlaceholderTestCases.php b/core/modules/big_pipe/tests/modules/big_pipe_test/src/BigPipePlaceholderTestCases.php
index d1ba348e1206..c376676a9699 100644
--- a/core/modules/big_pipe/tests/modules/big_pipe_test/src/BigPipePlaceholderTestCases.php
+++ b/core/modules/big_pipe/tests/modules/big_pipe_test/src/BigPipePlaceholderTestCases.php
@@ -88,11 +88,11 @@ class BigPipePlaceholderTestCases {
           'command' => 'insert',
           'method' => 'replaceWith',
           'selector' => '[data-big-pipe-placeholder-id="callback=Drupal%5CCore%5CRender%5CElement%5CStatusMessages%3A%3ArenderMessages&args%5B0%5D&token=_HAdUpwWmet0TOTe2PSiJuMntExoshbm1kh2wQzzzAA"]',
-          'data' => '<div data-drupal-messages>' . "\n" . ' <div role="contentinfo" aria-label="Status message" class="messages messages--status">' . "\n" . ' <h2 class="visually-hidden">Status message</h2>' . "\n" . ' Hello from BigPipe!' . "\n" . ' </div>' . "\n" . ' </div>' . "\n",
+          'data' => '<div data-drupal-messages>' . "\n" . ' <div role="contentinfo" aria-label="Status message">' . "\n" . ' <h2 class="visually-hidden">Status message</h2>' . "\n" . ' Hello from BigPipe!' . "\n" . ' </div>' . "\n" . '</div>' . "\n",
           'settings' => NULL,
         ],
       ];
-      $status_messages->embeddedHtmlResponse = '<div data-drupal-messages-fallback class="hidden"></div><div data-drupal-messages>' . "\n" . '    <div role="contentinfo" aria-label="Status message" class="messages messages--status">' . "\n" . '                  <h2 class="visually-hidden">Status message</h2>' . "\n" . '                    Hello from BigPipe!' . "\n" . '            </div>' . "\n" . '    </div>' . "\n";
+      $status_messages->embeddedHtmlResponse = '<div data-drupal-messages-fallback class="hidden"></div><div data-drupal-messages>' . "\n" . '  <div role="contentinfo" aria-label="Status message">' . "\n" . '              <h2 class="visually-hidden">Status message</h2>' . "\n" . '              Hello from BigPipe!' . "\n" . '          </div>' . "\n" . '</div>' . "\n";
     }
 
     // 2. Real-world example of HTML attribute value placeholder: form action.
diff --git a/core/modules/big_pipe/tests/src/Functional/BigPipeTest.php b/core/modules/big_pipe/tests/src/Functional/BigPipeTest.php
index 5624904e4a4e..99dadf3543e5 100644
--- a/core/modules/big_pipe/tests/src/Functional/BigPipeTest.php
+++ b/core/modules/big_pipe/tests/src/Functional/BigPipeTest.php
@@ -37,7 +37,7 @@ class BigPipeTest extends BrowserTestBase {
   /**
    * {@inheritdoc}
    */
-  protected $defaultTheme = 'classy';
+  protected $defaultTheme = 'stark';
 
   /**
    * {@inheritdoc}