Set default theme for LoadTest to be stark

tests/src/Functional/LoadTest.php

@@ -26,6 +26,13 @@ class LoadTest extends BrowserTestBase {
    */
   protected $user;
 
+  /**
+   * Set the default theme to use.
+   *
+   * @var \string
+   */
+  protected $defaultTheme = 'stark';
+
   /**
    * {@inheritdoc}
    */