Desktop styles for Header and Navigation in internal pages

![screen shot 2014-08-08 at 3 58 24 pm](https://cloud.githubusercontent.com/assets/662225/3862051/64951aba-1f36-11e4-8f33-9b20253107a1.png)

Wait, I believe we reverted that: https://github.com/thoughtbot/guides/pull/210