Originally posted by Ragnorak
The greasemonkey script "page recolorization" does part of what you want.
I use it mainly for changing white writing on black background to black on white.
Thanks for the link.
I think I may have found another way, though. I'd really like to use my own CSS-template (to gain full control over appearance). I thought about it and realized it should be possible to write my own templates and have them associated with different sites. I haven't figured out how yet (although this link seem promising - http://persistent.info/archives/2004/10/05/gmail-skinning - It's a bit outdated, but it should still work with ff*).
This way the developer(s) of RHP won't have to change anything. If they change the HTML, I'll just edit my RHP template.
*) The example in the link uses an id for the body to separate different sites. Very few sites have these ID:s, but I'm hoping to convince the maintainers of the sites I care to customize to do that. It's simple enough. RHP actually uses a class identifier, which would work for me as well.