That's just client side surely? I remembery daughter showed me an extension which turns every website you view in to a text by Snoop Dogg - if it was handled by an extension then the extension just needs to remember where you've been and over ride the css that's normally used surely?
Discussion
it was just a stupid idea for the online bookmark, if you have already visited the site and being able to change the color of the link already visited you can change it to the background color and not see it