Force a page to be rendered in admin theme In drupal 7 we could force a page to open in admin page, there are several way to do that in drupal 7 


with hook_admin_paths 

function hook_admin_paths() {
  $paths = array(
    'mymodule//add' => TRUE,
    'mymodule//edit' => TRUE,
  );
  return $paths;
}
with Admin theme module 
with ThemeKeys module 


Now I am looking for equivalent solution in Drupal 8, How can I force drupal to render a specified pages as 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.