GameDev.net is a large site and I haven't checked every page yet, so it is highly likely that you'll end up with some white text on white backgrounds or black text on black backgrounds. Unfortunately it seems that people are using inline colours more with the new system which I can't do much about through CSS, however !important I try to make the rule.
The stylesheet can be found here: classic-black.css. Firefox users can install Stylish and use benryves_classic-black-fixed-userstyle.css (many thanks to coldacid for this version and for fixing some bugs). I'm not sure how other browsers deal with user stylesheets (guides for IE indicated that the stylesheet would be loaded on every site, which isn't much use), I'm afraid, but in Opera you can right-click the page, select Edit site preferences and then select the stylesheet at the bottom of the Display tab.
For browsers that can't apply user stylesheets to specific websites, here's a bookmarklet that applies the stylesheet when clicked. Drag the link to your bookmarks, then visit a page on GameDev.net and click the link to apply the style. It's only temporary (and will be lost when you change page) but it is at least a preview.
If anyone tries this, I'd be happy to hear any feedback (especially bug reports!) I didn't write all of the CSS by hand; certain rules were set up in an INI file which is then "compiled" with a little command-line tool (usage: "CSS Compiler" classic-black.ini classic-black.css). The tool has no error handling whatsoever and will keel over if you so much as look at it funny but it sped up the process quite considerably.