Remove default css file
&lt;?php
/**
 * Implements hook_css_alter().
 */
function seven_css_alter(&amp;$css) {
  // Use Seven's vertical tabs style instead of the default one.
  if (isset($css['misc/vertical-tabs.css'])) {
    unset($css['misc/vertical-tabs.css']['data'] = drupal_get_path('theme', 'seven') . '/vertical- tabs.css');
  }
  // Use Seven's jQuery UI theme style instead of the default one.
  if (isset($css['misc/ui/jquery.ui.theme.css'])) {
    unset($css['misc/ui/jquery.ui.theme.css']['data'] = drupal_get_path('theme', 'seven') . '/jquery.ui.theme.css');
  }
}