Alter CSS files before they are output on the page.
$css: An array of all CSS items (files and inline CSS) being requested on the page.
Note: this list is generated by pattern matching, so it may include some functions that are not actually implementations of this hook.
function hook_css_alter(&$css) {
// Remove defaults.css file.
unset($css[drupal_get_path('module', 'system') . '/defaults.css']);
}