DbUpdateNegotiator.php

<?php

namespace Drupal\system\Theme;

use Drupal\Core\Config\ConfigFactoryInterface;
use Drupal\Core\Routing\RouteMatchInterface;
use Drupal\Core\Site\Settings;
use Drupal\Core\Theme\ThemeNegotiatorInterface;

/**
 * Sets the active theme for the database update pages.
 */
class DbUpdateNegotiator implements ThemeNegotiatorInterface {
    
    /**
     * The config factory.
     *
     * @var \Drupal\Core\Config\ConfigFactoryInterface
     */
    protected $configFactory;
    
    /**
     * Constructs a DbUpdateNegotiator.
     *
     * @param \Drupal\Core\Config\ConfigFactoryInterface $config_factory
     *   The config factory.
     */
    public function __construct(ConfigFactoryInterface $config_factory) {
        $this->configFactory = $config_factory;
    }
    
    /**
     * {@inheritdoc}
     */
    public function applies(RouteMatchInterface $route_match) {
        return $route_match->getRouteName() == 'system.db_update';
    }
    
    /**
     * {@inheritdoc}
     */
    public function determineActiveTheme(RouteMatchInterface $route_match) {
        $custom_theme = Settings::get('maintenance_theme', 'seven');
        if (!$custom_theme) {
            $config = $this->configFactory
                ->get('system.theme');
            $custom_theme = $config->get('default');
        }
        return $custom_theme;
    }

}