Improve styling of out-of-the-box navbar-toggle element on an inverse navbar

focus + hover:
![screen shot 2014-11-11 at 3 32 11 pm](https://cloud.githubusercontent.com/assets/111218/5002645/f43f4728-69b7-11e4-8dde-e1c30f1463d7.png)

active:
![screen shot 2014-11-11 at 3 31 55 pm](https://cloud.githubusercontent.com/assets/111218/5002644/f43d4b58-69b7-11e4-9be5-e07718450e4d.png)

Fixes #895? @mejackreed ?