How to set admin theme for a route? According to Drupal handbook route definition accepts parameter called _theme. Unfortunately this parameter is documented as "TODO".

Can this be used to force admin theme for the route? Is it possible to inject config parameter with admin theme name here?

It appears that other solution is to use Theme negotiator however the _theme parameter looks very tempting and way simpler to use.
You're looking for the _admin_route option:

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