How to *toggle* usercontent.css? Or have multiple style sheets.

I'm trying to switch from Opera to Mozilla.  I've been using opera for 
its ability to easily switch between author and user mode (different css).

I can get mozilla to use my (black background) style sheet by 
overwriting userContent.css.  But then I cannot find out how to turn it 
off while browsing (without restarting mozilla).

Can I force multiple to options appear under View -> Use Style?  (Or can 
one style sheet somehow define multiple styles to switch between?)

Thanks,
--Ryan