Trouble creating custom admin css

I created a small plugin to help me load custom css for the admin area but if is not working. The plugin is just the function :

function wt_custom_admin() {
    $url = get_option('siteurl');
    $url = $url . '/wp-content/plugins/wt-pretty-admin/wp-admin.css';
	}
	add_action('admin_head', 'wt_custom_admin');

and the css file with overrides of the default admin styles.

Can anyone see why this is not working ?

Oh.. nevermind.. I forgot to actually echo the stylesheet in the function. For some reason in the documentation I was reading this part was struck out.

http://codex.wordpress.org/Creating_Admin_Themes

Is there a better way to do this?