Change the theme used for the user reset password page
You can use a RouteSubscriber to do that but it will also affect your user edit form and render it with the default site theme since it uses the same route name. The RouteSubscriber can't alter a route based on a query string parameter such as pass-reset-token
yourmodule/yourmodule.services.yml
services:
  yourmodule.routesubscriber:
    class: Drupal\yourmodule\Routing\YourmoduleRouteSubscriber
    tags:
      - { name: event_subscriber, priority: 1 }

yourmodule/src/Routing/YourmoduleRouteSubscriber.php
&lt;?php

namespace Drupal\yourmodule\Routing;

use Drupal\Core\Routing\RouteSubscriberBase;
use Symfony\Component\Routing\RouteCollection;

/**
 * Custom route subscriber.
 */
class YourmoduleRouteSubscriber extends RouteSubscriberBase {

  /**
   * {@inheritdoc}
   */
  protected function alterRoutes(RouteCollection $collection) {
    if ($route = $collection-&gt;get('entity.user.edit_form')) {
      $options = $route-&gt;getOptions();
      $options['_custom_theme'] = 'yourthemename';
      $route-&gt;setOptions($options);
    }
  }

}