Theme Negotiators - Switch theme dynamically

Create a custom module (here : example)

For active themes by route you first have to define a service in your $module.services.yml file:

On example.services.yml

Add:

services:

  theme.negotiator.example:

    class: Drupal\example\Theme\ExampleNegotiator

    tags:

      - { name: theme_negotiator, priority: 0 }

and then use that new defined service class:

Create a class ExampleNegotiator on example/src/Theme With:

namespace Drupal\example\Theme;

use Drupal\Core\Theme\ThemeNegotiatorInterface;

use Drupal\Core\Routing\RouteMatchInterface;

class ExampleNegotiator implements ThemeNegotiatorInterface {

  /**

   * {@inheritdoc}

   */

  public function applies(RouteMatchInterface $route_match) {

    // Use this theme on a certain route.

    return $route_match->getRouteName() == 'example_route_name';

  }

  /**

   * {@inheritdoc}

   */

  public function determineActiveTheme(RouteMatchInterface $route_match) {

    // Here you return the actual theme name.

    return 'stark';//The theme name

  }

}