Ensure container backgrounds expand to full width on scroll

This fixes the issue raised in #297

I couldn't figure out how to run the static site locally, but making the same edits in dev tools shows the fix on firefox/safari/chrome.

![sub messaging for the web 2014-05-12 21-29-38 2014-05-12 21-30-21](https://cloud.githubusercontent.com/assets/78225/2949917/3e4f9892-da14-11e3-93cb-e2ab1031fc3b.png)