I've never gotten a virus on Android so long as you keep it within the play store. I've heard of Spy ware though. If you want themes and are concerned download an anti virus for your phone prior to installing themes.
Ok, what is with the sudden paranoia in your 3 posts? Did you load/install something from outside of the PlayStore? Why not just install an anti virus program to assuage your fears? There are plenty of good and free ones.