I had this setup before but forget how i did it and where the documentation is.
Basically what i want to do is i have 2 skins in the skins directory of my roundcube, and i want to automatically use skin 'larry' on desktop and another we will call it 'mobileskin' on mobile devices. i did this before with plugin rcs_skins and cant remember how to do it, hoping someone here can help me.
i am using iredmail 0.9.4 that uses roundcube. i have a config file in /opt/www/roundcubemail/config/ called config.inc.php where i can specify plugins 'rcs_skins' is definded there, and in the roundcube skins folder i have both skins /opt/www/roundcubemail/skins/
You could look at the mobile plugin.