Exclude Drupal system CSS for logged-out users

In your template.php, you can exclude Drupal’s system CSS files, or any CSS file that wasnt added by your theme (this includes CSS added by modules) using this PHP snippet:

function YOURTHEME_css_alter(&$css) {
  if (!user_is_logged_in()) {
    foreach ($css as $key => $value) {
      if ($value['group'] != CSS_THEME) {
        $exclude[$key] = FALSE;
      }
    }
    $css = array_diff_key($css, $exclude);
  }
}