Adding styles through the API

<?php
$inc_path = drupal_get_path('theme', 'your-theme-name') . '/my-styles.css';
$file = realpath(".") ."/". $inc_path;
if(file_exists($file)) {
   drupal_add_css($inc_path , 'theme', 'all', FALSE);
   $vars['css']['all']['theme'][$inc_path] = 1; // this is what makes it work!
   $vars['styles'] = drupal_get_css();       
}
?>