class ThemeAccessCheck

class ThemeAccessCheck implements AccessInterface {
    
    /**
     * The theme handler.
     *
     * @var \Drupal\Core\Extension\ThemeHandlerInterface
     */
    protected $themeHandler;
    
    /**
     * Constructs a \Drupal\Core\Theme\Registry object.
     *
     * @param \Drupal\Core\Extension\ThemeHandlerInterface $theme_handler
     *   The theme handler.
     */
    public function __construct(ThemeHandlerInterface $theme_handler) {
        $this->themeHandler = $theme_handler;
    }
    
    /**
     * 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) {
        $themes = $this->themeHandler
            ->listInfo();
        return !empty($themes[$theme]->status);
    }

}