Darken side margins

/cc @vilderness

### Before

![screen shot 2014-08-06 at 12 06 58](https://cloud.githubusercontent.com/assets/85783/3825904/f184c3b4-1d59-11e4-910c-67d0b946b181.PNG)


### After

![screen shot 2014-08-06 at 12 06 40](https://cloud.githubusercontent.com/assets/85783/3825905/f452159c-1d59-11e4-8c6b-8b9b90279a92.PNG)


![ ](http://media1.giphy.com/media/9EmYYw6o9cmsM/giphy.gif)