Foren / Cotonti / Support / Redirect to Mobile theme

First you need to distinct a user of a mobile device somehow. Either by the URL (i.e. use example.com/mobile or m.example.com for mobile devices) or by User-Agent (which are plenty of I suppose). Then, supposing that you use m.example.com for mobiles, you can write in your config.php:

$cfg['defaultskin'] = $_SERVER['HTTP_HOST'] == 'm.example.com' ? 'wap_skin' : 'normal_skin';