Found this handy little bookmarklet on a website, if you click it, or bookmark it and use it, it turns the web page you’re looking at to black text on a white background. Obviously, any images or heavily CSSed elements may still render in the original colours and vanish, but it’s handy if you truly can’t stand the colours a site has.

Transmogrify Website Colours
(Right click and choose Bookmark, works in Firefox, can’t promise it’ll work in other browsers, you can then either use it from your bookmarks menu, or whack it onto a button bar or something).