Remove color definitions for code blocks

Fixes colors not being consistent in code blocks, by removing explicit color definitions for code blocks.

Before:
![screen shot 2014-01-27 at 14 45 47](https://f.cloud.github.com/assets/211284/2009153/bc6f4d0c-8759-11e3-86ac-5343372529d8.png)

After:
![screen shot 2014-01-27 at 14 45 33](https://f.cloud.github.com/assets/211284/2009156/bf937620-8759-11e3-8358-2f67e404409a.png)