ThemeManager::initTheme

protected ThemeManager::initTheme(RouteMatchInterface $route_match = NULL)

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);
}