There are ways to do it, but it WILL affect every website in the same manner.
Under IE8 (and I believe 7. If you're still on 6 please upgrade to 8!):
Tools > Internet Options
At the bottom of the General tab is Appearance, with a button labeled "colors", once selected there is a link "How to ignore preset colors". Hit that link and read on. Saves me a whack of typing!
Under Firefox:
Tools > Options > Content > Colors
Select "Use System Colors" and de-select "Allow Web pages to choose their own colors", and adjust by using the pull down menus beside each item.
CAUTION this will affect ALL websites.
I have to say, I've gotten used to the colors, though I wish they would make different 'skins' available to us!