Use a text color of black to make diff readable

Pretty simple. Add .black to text to make it legible.

Example before:
![screen shot 2013-11-08 at 12 46 37 pm](https://f.cloud.github.com/assets/525011/1502633/bccb4af8-489d-11e3-96c7-2beecee15762.png)

Example after:
![screen shot 2013-11-08 at 8 39 14 pm](https://f.cloud.github.com/assets/525011/1505630/e6c67c3c-48df-11e3-9a3b-285e507119d9.png)

Example after on light background:
![screen shot 2013-11-08 at 8 41 01 pm](https://f.cloud.github.com/assets/525011/1505631/0f9b0128-48e0-11e3-8b56-defae5fdcb25.png)

closes #397