Fixed header background on large screens

Hey,
On large screen there is an issue with header background. I'm attaching screenshots before and after my fix. Hope you like it.

Best regards,
Oskar
![before](https://cloud.githubusercontent.com/assets/5034850/4034972/6af91166-2c8a-11e4-974c-57d1c8160530.png)
![Uploading after.png . . .]()


should we consider http://css-tricks.com/perfect-full-page-background-image/ ?