Have configuration form use a different theme than the currently set Admin theme
You can control this in the route definition.

If you give non admin users access to the form they will see it in a regular theme:

my_settings:
  path: '/admin/config/system/my_settings'
  defaults:
    _form: 'Drupal\my_settings\Form\SettingsForm'
  requirements:
    _permission: 'access content'


If you want to have admins see the form in a regular theme then move the path outside of /admin:

my_settings:
  path: '/other-path/my_settings'
  defaults:
    _form: 'Drupal\my_settings\Form\SettingsForm'
  requirements:
    _permission: 'administer site configuration'


Or do both at the same time.

Details:

If you give a non admin user permission for an admin route, then the form will be displayed in the regular theme, because the admin theme is only accessible for admin users or users who have the "administration theme" permission.

Whether a route is considered as admin route is controlled by the route option _admin_route. If this option is not set, it is added by AdminRouteSubscriber::alterRoutes to all routes with an admin path:

  protected function alterRoutes(RouteCollection $collection) {
    foreach ($collection-&gt;all() as $route) {
      if (strpos($route-&gt;getPath(), '/admin') === 0 &amp;&amp; !$route-&gt;hasOption('_admin_route')) {
        $route-&gt;setOption('_admin_route', TRUE);
      }
    }
  }


If you want to override this you can set the option to false:

  options:
    _admin_route: FALSE