class ThemeAccessCheck

class ThemeAccessCheck implements AccessInterface {
  public function __construct(protected array $themes) {
  }
  
  /**
   * Checks access to the theme for routing.
   *
   * @param string $theme
   *   The name of a theme.
   *
   * @return \Drupal\Core\Access\AccessResultInterface
   *   The access result.
   */
  public function access($theme) {
    // Cacheable until the theme settings are modified.
    return AccessResult::allowedIf($this->checkAccess($theme))
      ->addCacheTags([
      'config:' . $theme . '.settings',
    ]);
  }
  
  /**
   * Indicates whether the theme is accessible based on whether it is installed.
   *
   * @param string $theme
   *   The name of a theme.
   *
   * @return bool
   *   TRUE if the theme is installed, FALSE otherwise.
   */
  public function checkAccess($theme) {
    return isset($this->themes[$theme]);
  }

}