You can also use the Link CSS module to achieve this.
Or, in your own custom module, use hook_css_alter
as described here: http://e9p.net/disable-import-tags-stylesheets-while-development-drupal
/**
* Implements hook_css_alter().
*/
function mymodule_css_alter(&$css) {
$preprocess_css = variable_get('preprocess_css', TRUE);
if (!$preprocess_css) {
// For each item, don't allow preprocessing to disable @import.
foreach ($css as &$item) {
if (file_exists($item['data'])) {
$item['preprocess'] = FALSE;
}
}
}
}