class ThemeNegotiatorWrapper

class ThemeNegotiatorWrapper extends ThemeNegotiator {
    
    /**
     * @var \Drupal\Core\Theme\ThemeNegotiatorInterface
     */
    private $negotiator;
    
    /**
     * {@inheritdoc}
     */
    public function determineActiveTheme(RouteMatchInterface $route_match) {
        // This method has changed in Drupal 8.4.x, to maintain compatibility with
        // Drupal 8.3.x we check the existence or not of the classResolver
        // property.
        // TODO: remove this logic when we decide to drop Drupal 8.3.x support.
        if (property_exists($this, 'classResolver')) {
            $classResolver = $this->classResolver;
            $negotiators = $this->negotiators;
        }
        else {
            $classResolver = \Drupal::classResolver();
            $negotiators = $this->getSortedNegotiators();
        }
        foreach ($negotiators as $negotiator_id) {
            if (property_exists($this, 'classResolver')) {
                $negotiator = $classResolver->getInstanceFromDefinition($negotiator_id);
            }
            else {
                $negotiator = $negotiator_id;
            }
            if ($negotiator->applies($route_match)) {
                $theme = $negotiator->determineActiveTheme($route_match);
                if ($theme !== NULL && $this->themeAccess
                    ->checkAccess($theme)) {
                    $this->negotiator = $negotiator;
                    return $theme;
                }
            }
        }
    }
    
    /**
     * @return \Drupal\Core\Theme\ThemeNegotiatorInterface
     */
    public function getNegotiator() {
        return $this->negotiator;
    }
    
    /**
     * Returns the sorted array of theme negotiators.
     *
     * @return array|\Drupal\Core\Theme\ThemeNegotiatorInterface[]
     *   An array of theme negotiator objects.
     *
     * TODO: remove this method when we decide to drop Drupal 8.3.x support.
     */
    protected function getSortedNegotiators() {
        if (!isset($this->sortedNegotiators)) {
            // Sort the negotiators according to priority.
            krsort($this->negotiators);
            // Merge nested negotiators from $this->negotiators into
            // $this->sortedNegotiators.
            $this->sortedNegotiators = [];
            foreach ($this->negotiators as $builders) {
                $this->sortedNegotiators = array_merge($this->sortedNegotiators, $builders);
            }
        }
        return $this->sortedNegotiators;
    }

}