How to load css files for themes with Pill's Template library

[eluser]Unknown[/eluser]
Can someone tell me howto load css files for each theme?

Example:
themes/default/css/style.css
themes/admin/css/style.css

I've tried,

Code:

->append_metadata( css('style.css') )
AND
->append_metadata( theme_css('style.css') )

Loading css files from modules works fine.

Code:

->append_metadata( css('style.css', 'module_name') )

I've changed asset_dir and other stuff in application/config/asset.php to themes/

Code:

$config['asset_dir'] = 'themes/';