gitweb.css

I've been customizing a gitweb server to fit with the rest of our project pages (the result is http://git.racket-lang.org/). This was relatively easy to do except for a few places where gitweb.css specifies formatting for generic tags like `body', `table', and `td', which messed up our header.

Maybe it makes sense to localize these styles to to gitweb specific classes? (I know that I can just use my own css, but the file is big enough that I prefer avoiding manually merging in updates.)