How to enqueue CSS to administrator side via module

I have a custom Drupal module and I would like to enqueue its CSS to the administrator side. I've tried to add it to the .info.yml file.

css:
  theme:
    admin-styles.css: {}

I also implemented hook_page_attachments() in the module as described in Custom module for adding CSS file in admin section.

function admin_css_page_attachments(array &$attachments) {
  $is_admin = \Drupal::service('router.admin_context')->isAdminRoute();
  if ($is_admin) {
    $attachments['#attached']['library'][] = 'admin-styles.css';
  }
}

I still don't see the CSS file in the source code of the administrator panel.

You need to define your library in *.libraries.yml:

example_theme.libraries.yml:

my_library:
  css:
    theme:
      admin-styles.css: {}

Then you can add this library using example_theme/my_library.