Best practice for adding CSS to admin core theme?

Thanks everyone for respond. Since there was nothing to review and original poster hasn't asked for additional help in 7 years think this is good to close.

Again thanks!

You can also use the Asset Injector module to add css to your admin theme: https://www.drupal.org/project/asset_injector →

Or with this short custom module: https://gorannikolovski.com/snippet/add-css-and-js-admin-pages