Stretch background color on very wide call stacks

Fix the call stack printer so that when it is very wide, the background colors no longer cut off at the original width of the browser window.

Before: 

![Screen Shot 2013-02-14 at 2 16 21 PM](https://f.cloud.github.com/assets/23479/158342/8319f296-76e3-11e2-89be-f0c3e6b44568.png)


:sob:

After: 

![Screen Shot 2013-02-14 at 2 16 53 PM](https://f.cloud.github.com/assets/23479/158343/8b6b1a10-76e3-11e2-8b4f-cc5333f7d34b.png)

:heart_eyes: 


Checked in Chrome, Firefox, and Safari on MacOS.  

:thumbsup:?