global .vimrc?

I set bg=dark in my ~/.vimrc file and everything is fine. The problem is
that when I run "su -" and then run "vi" on a file I'm back to dark blue
text on a black background. I looked for a global .vimrc file but can't
find one. Any pointers?

Thanks.

--
Bob Holtzman
"If you think you're getting free lunch,
......check the price of the beer!"