Your code doesn't have to look so sour. I've collected a list of Xcode themes (and fonts) a while ago. Many more themes have been submitted in the meanwhile so if you have a theme you'd like to add to the list, just let me know!
by Zeno Rocha
Added by vencentle
One of the few light themes that made the list.
Inspired by the famous Railscast TextMate theme.
Available for all kind of editors on GitHub.
Nice dark variant.
My absolute favorite!
Thanks for the tip Damian Carrillo.
My favorite. Shipped with Mac OS.
A great Menlo alternative that comes with three different line height options. Recommended if you prefer a little bit of air between your lines.
Great font, also shipped with Mac OS.
Provided with Coda.
Adobe's open source font. Recommended!
Find and install ThemeInstaller app from the Mac App Store. Open theme you want to install and it's done. Themes are visible after Xcode is restarted.