From http://drupal.stackexchange.com/questions/4289/how-to-override-some-admin-theme-css-rules

override_admin_theme_css.php

<?php
// In custom module
function mymodule_preprocess_html(&$variables) {
// Add conditional stylesheets for admin pages on admin theme.
  if (arg(0)=="admin") {
    // reference your current admin theme  
    $theme_path = drupal_get_path('theme', 'commerce_kickstart_admin');
    // reference your own stylesheet    
    drupal_add_css(drupal_get_path('module', 'mymodule') . '/css/mymodule.css', array('weight' => CSS_THEME));
  }
}