ThemeNegotiatorInterfaceExample.php

<?php

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

/**
 * Class CustomThemeNegotiator.
 *
 * @package Drupal\bixal_theme_helpers
 */
class CustomThemeNegotiator implements ThemeNegotiatorInterface {

  /**
   * Whether this theme negotiator should be used to set the theme.
   *
   * @param \Drupal\Core\Routing\RouteMatchInterface $route_match
   *   The current route match object.
   *
   * @return bool
   *   TRUE if this negotiator should be used or FALSE to let other negotiators
   *   decide.
   */
  public function applies(RouteMatchInterface $route_match) {
    return \Drupal::service('path.matcher')->isFrontPage();
  }

  /**
   * Determine the active theme for the request.
   *
   * @param \Drupal\Core\Routing\RouteMatchInterface $route_match
   *   The current route match object.
   *
   * @return string|null
   *   The name of the theme, or NULL if other negotiators, like the configured
   *   default one, should be used instead.
   */
  public function determineActiveTheme(RouteMatchInterface $route_match) {
    return 'custom_theme';
  }
}