You need to know some CSS to do such things in general. In
Greasemonkey the whole script should be:
GM_addStyle("body { color: black !important; background-color: white !
important; }");
see
http://wiki.greasespot.net/GM_addStyle
(the "!important"s may not be needed)
The above should work if your target page only styles its body. If
internal parts such as divs are also colored styled, you may have to
restyle them too.
Your application really doesn't need Greasemonkey. It might be better
to do it in the Stylish extension because that applies styles before
the page is loaded, not after as Greasemonkey does which sometimes
causes an annoying flash.
https://addons.mozilla.org/en-US/firefox/addon/2108
--
Which conspiracy theory do you believe?
The Government's Official Conspiracy Theory Story,
or one of the alternative conspiracy theories?
Demand Truth and Justice! http://www.Chico911Truth.org/