ContextualLinksNegotiator.php

<?php

namespace Drupal\contextual\Theme;

use Drupal\Core\Config\ConfigFactoryInterface;
use Drupal\Core\Routing\RouteMatchInterface;
use Drupal\Core\Theme\ThemeNegotiatorInterface;
use Drupal\Core\Extension\ThemeHandlerInterface;
use Symfony\Component\HttpFoundation\RequestStack;

/**
 * Set the theme according to the parameter passed to the controller.
 */
final class ContextualLinksNegotiator implements ThemeNegotiatorInterface {
  public function __construct(protected readonly RouteMatchInterface $route_match, protected readonly RequestStack $requestStack, protected readonly ThemeHandlerInterface $themeHandler, protected readonly ConfigFactoryInterface $configFactory) {
  }
  public function applies(RouteMatchInterface $route_match) : bool {
    return $route_match->getRouteName() === 'contextual.render';
  }
  public function determineActiveTheme(RouteMatchInterface $route_match) : string {
    $request = $this->requestStack
      ->getCurrentRequest();
    $theme = $request?->query->get('theme', '') ?? '';
    if ($this->themeHandler
      ->themeExists($theme)) {
      return $theme;
    }
    return $this->configFactory
      ->get('system.theme')
      ->get('default');
  }

}