Custom module for adding CSS file in admin section
Unless you use the same theme for front and backend, that's not the right approach. 

If something transcends the theme layer, e.g. you have functionality that you need to apply to two separate themes, it belongs in a module.

See Adding stylesheets (CSS) and JavaScript (JS) to a Drupal 8 module for instructions to actually accomplish that, specifically the section on Attaching a library to all (or a subset of) pages which contains a contrived code example:

function contextual_page_attachments(array &amp;$attachments) {
  if (!\Drupal::currentUser()-&gt;hasPermission('access contextual links')) {
    return;
  }

  $attachments['#attached']['library'][] = 'contextual/drupal.contextual-links';
}