Your code doesn't have to look so sour. I've collected a list of Xcode themes and fonts that you can install. Read my blog post which also contains a collection of fonts.

Xcode theme's

Darkcloud

EGO V2

by Enormego

Kelly's

Monokai

Night

Quiet Light

The only light theme that made the list.

Railscast

Inspired by the famous Railscast TextMate theme.

Sidewalk Chalk

Solarized Light

Yozora

My absolute favorite!

Installation

1. Copy the theme(s) to the Xcode's FontAndColorThemes folder (~/Library/Developer/Xcode/UserData/FontAndColorThemes/).
2. Restart Xcode.
3. Enjoy.