> Theme is stored with your user account, so at the moment this wouldn't be possible.
It should be relatively easy to store 2 themes for mobile/pc and use them depending on the user agent.
Edit: just parse user agent for words like android, mobile, tablet, iphone, ipad. Use mobile theme if found anything or pc theme otherwise. You don't even need to parse UA every request. Just very first one then use single theme for the session. IIRC it expires after 15 (or was it 10?) minutes. So in the worst case you will do it 4 (6?) times per hour for every user.