If you like doing things the harder but more precise way, get "user javascript and css" and edit it yourself. White is #FFFFFF and black is #000000.
body {
background-color: #FFFFFF;
font-color: #000000;
}
That should work but I haven't tested it. If not, hit f12 and inspect the elements to get the tag they are using and add it if needed.
I think they have the extension for FF and chrome, but if not there are many extensions that let you edit a pages css and any of them will work.
i like Stylus (addons.mozilla.org) - excellent extension by an ethical developer
(post is archived)