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!

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Xcode themes

Darkcloud

Dracula

by Zeno Rocha

EGO V2

by Enormego

ElChacho

by OrlandoAleman

Flatland

by mduvall

Kelly's

Monokai

Night

Quiet Light

One of the few light themes that made the list.

Railscast

Inspired by the famous Railscast TextMate theme.

Sidewalk Chalk

Solarized Light

Tomorrow

Available for all kind of editors on GitHub.

Tomorrow Dark

Nice dark variant.

Yozora

My absolute favorite!

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Fonts

Bitstream Vera Sans (download)

Consolas (instructions to download on Mac)

Courier Prime (download)

Thanks for the tip Damian Carrillo.

Droid Sans Mono (download)

Inconsolata (download)

Menlo

My favorite. Shipped with Mac OS.

Meslo (download)

A great Menlo alternative that comes with three different line height options. Recommended if you prefer a little bit of air between your lines.

Monaco

Great font, also shipped with Mac OS.

Panic Sans (install instructions)

Provided with Coda.

Source Code Pro (download)

Adobe's open source font. Recommended!

Ubuntu Mono (install instructions)

Installation

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