container->get('theme_installer')->install(['classy']); $this->container->get('config.factory') ->getEditable('system.theme') ->set('default', 'classy') ->save(); // Clear the theme registry. $this->container->set('theme.registry', NULL); } /** * Test the classy theme. */ public function testClassyTheme() { \Drupal::messenger()->addError('An error occurred'); \Drupal::messenger()->addStatus('But then something nice happened'); $messages = [ '#type' => 'status_messages', ]; $this->render($messages); $this->assertNoText('custom-test-messages-class', 'The custom class attribute value added in the status messages preprocess function is not displayed as page content.'); } }