getMasterRouteMatch example

  protected function initTheme(RouteMatchInterface $route_match = NULL) {
    // Determine the active theme for the theme negotiator service. This includes     // the default theme as well as really specific ones like the ajax base theme.     if (!$route_match) {
      $route_match = \Drupal::routeMatch();
    }
    if ($route_match instanceof StackedRouteMatchInterface) {
      $route_match = $route_match->getMasterRouteMatch();
    }
    $theme = $this->themeNegotiator->determineActiveTheme($route_match);
    $this->activeTheme = $this->themeInitialization->initTheme($theme);
  }

  /** * {@inheritdoc} * * @todo Should we cache some of these information? */
  public function alterForTheme(ActiveTheme $theme, $type, &$data, &$context1 = NULL, &$context2 = NULL) {