Add background color to panel, make usable with dark dev tools themes

### Whats Up?

When using a dark dev tools theme, the background of rails panel is transparent, so you get black text on a dark background rendering it unusable.

### What changed?

 - Add a class to the rails panel body element, to avoid naming collisions
 - Give the rails panel body element a background color of `#fff`

### Example of issue 

Before:

![screen shot 2014-12-02 at 8 29 25 am](https://cloud.githubusercontent.com/assets/1007571/5266853/4e1b8fe6-7a01-11e4-8d98-b5868da8cef3.png)

After:

![screen shot 2014-12-02 at 8 46 48 am](https://cloud.githubusercontent.com/assets/1007571/5266858/55e1763c-7a01-11e4-9a97-c4c839db23e2.png)

This is an integral chrome extension when working with rails, seriously great work.