Change css for induvidual users

Hi,

I have some problems with a plugin I'm writing.

I want the user to be able to change between diffrent themes (actually it's just different directories with modified css files taken from views/default/css/elements), but I can't get simplecache working.

I have the logic in place, so a user can select between diffrent themes.

If one user changes theme, all users get's that theme.

So how do I switch between cached css's on a user basis?

Regards

PS: I need simplecache for performance!