AdminDemoNegotiator.php

<?php

namespace Drupal\block\Theme;

use Drupal\Core\Routing\RouteMatchInterface;
use Drupal\Core\Theme\ThemeNegotiatorInterface;

/**
 * Negotiates the theme for the block admin demo page via the URL.
 */
class AdminDemoNegotiator implements ThemeNegotiatorInterface {
    
    /**
     * {@inheritdoc}
     */
    public function applies(RouteMatchInterface $route_match) {
        return $route_match->getRouteName() == 'block.admin_demo';
    }
    
    /**
     * {@inheritdoc}
     */
    public function determineActiveTheme(RouteMatchInterface $route_match) {
        // We return exactly what was passed in, to guarantee that the page will
        // always be displayed using the theme whose blocks are being configured.
        return $route_match->getParameter('theme');
    }

}