class ThemeEarlyInitializationTest

class ThemeEarlyInitializationTest extends BrowserTestBase {
  
  /**
   * Modules to enable.
   *
   * @var array
   */
  protected static $modules = [
    'theme_test',
  ];
  
  /**
   * {@inheritdoc}
   */
  protected $defaultTheme = 'starterkit_theme';
  
  /**
   * Tests that the theme system can generate output in a request listener.
   */
  public function testRequestListener() {
    $this->drupalGet('theme-test/request-listener');
    // Verify that themed output generated in the request listener appears.
    $this->assertSession()
      ->responseContains('Themed output generated in a KernelEvents::REQUEST listener');
    // Verify that the default theme's CSS still appears even though the theme
    // system was initialized early.
    $this->assertSession()
      ->responseContains('starterkit_theme/css/components/action-links.css');
  }

}