How to override some admin theme CSS rules?
I did it just using:

function mymodule_preprocess_html(&amp;$variables) {

  global $theme;

  if ($theme === variable_get('admin_theme', 'seven')) {

    // Reference your custom stylesheet.
    drupal_add_css(drupal_get_path('module', 'mymodule') . '/css/mymodule.css', array('weight' =&gt; CSS_THEME));
  }
}


That's a bit more readable for me.