MobileCube is presently the only theme I know of that works with the current [0.3] Roundcube release that is still being actively supported.
Someone recently updated MVISION v2 to work on RoundCube 0.3 however it's no longer actively supported so I can't really recommend it.
There is also a group (which includes the developer of MobileCube) working on a new theme project which is based on Roundcube called CrystalWebMail but very few details are know. They have a countdown timer on a newly created website (
UNDER CONSTRUCTION ) and that's about all the info available at present.
Martian