Force a page to be rendered in admin theme
You can add the _admin_route option to a route:

aggregator.feed_add:
  path: '/aggregator/sources/add'
  defaults:
    _controller: '\Drupal\aggregator\Controller\AggregatorController::feedAdd'
    _title: 'Add feed'
  requirements:
    _permission: 'administer news feeds'
  options:
    _admin_route: TRUE


You can also implement your own negotiator (See \Drupal\user\Theme\AdminNegotiator) or add that option to routes dynamically in a request subscriber.