tests/src/Functional/BigPipeSessionlessRegressionTest.php | 5 +++++
 tests/src/Functional/BigPipeSessionlessTest.php           | 5 +++++
 2 files changed, 10 insertions(+)

diff --git a/tests/src/Functional/BigPipeSessionlessRegressionTest.php b/tests/src/Functional/BigPipeSessionlessRegressionTest.php
index c14ce5e..b8e4bc7 100644
--- a/tests/src/Functional/BigPipeSessionlessRegressionTest.php
+++ b/tests/src/Functional/BigPipeSessionlessRegressionTest.php
@@ -16,6 +16,11 @@ use Drupal\Tests\BrowserTestBase;
  */
 class BigPipeSessionlessRegressionTest extends BrowserTestBase {
 
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
   /**
    * {@inheritdoc}
    */
diff --git a/tests/src/Functional/BigPipeSessionlessTest.php b/tests/src/Functional/BigPipeSessionlessTest.php
index dcc08a0..c44b07e 100644
--- a/tests/src/Functional/BigPipeSessionlessTest.php
+++ b/tests/src/Functional/BigPipeSessionlessTest.php
@@ -23,6 +23,11 @@ use Drupal\Core\Url;
  */
 class BigPipeSessionlessTest extends BigPipeTest {
 
+  /**
+   * {@inheritdoc}
+   */
+  protected $defaultTheme = 'stark';
+
   /**
    * Modules to enable.
    *