Add CSS and JS to Admin pages

Adding CSS or JS to admin pages is easy. Just create a new library, create CSS and JS files, and then attach the library in the hook_page_attachments() hook.

MY_MODULE.libraries.yml

admin_styling:
  version: VERSION
  css:
    theme:
      css/admin-styling.css: {}
  js:
    js/admin-script.js: {}

MY_MODULE.module

/**
 * Implements hook_page_attachments().
 */
function MY_MODULE_page_attachments(array &$attachments) {
  if (\Drupal::service('router.admin_context')->isAdminRoute()) {
    $attachments['#attached']['library'][] = 'MY_MODULE/admin_styling';
  }
}