I usually work at nights and tend to paint the most frequented sites with a la Batman.
GitHub is a giant place and I am in no rush to make it perfect, but it's to a point where almost everything is nice and with few bugs.
I use it with Stylebot which is great (Chrome). There's also Stylish which works in Chrome and Firefox and I'm sure there are other ways to apply stylesheets in other browsers.
If you like this idea but don't love my personal CSS, take a look here, there are more to choose from.
PS: The file size info on the screencap is from another script of mine (userscript). I'll upload soon here.