How do I override the admin theme in my own theme?
You can add admin CSS from a module hook.  Replace XXX with the module name.

1 Put your CSS in css/extra.admin.css

2 Declare a library by creating XXX.libraries.yml

extra.admin:
  css:
    theme:
      css/extra.admin.css: {}


3 Create a hook to load the library.

/**
 * Implements hook_page_attachments().
 */
function XXX_page_attachments(array &amp;$attachments) {
  $theme = \Drupal::theme()-&gt;getActiveTheme()-&gt;getName();
  if ($theme == 'seven') {
    $attachments['#attached']['library'][] = 'XXX/extra.admin';
  }
}