Update styles

Make quizzes in gray gitbook style
![selection_016](https://cloud.githubusercontent.com/assets/2205407/4607676/ba6dc67a-5259-11e4-9aab-a4126278debb.png)
![selection_015](https://cloud.githubusercontent.com/assets/2205407/4607675/b95b4708-5259-11e4-8a62-73d0cf6346a3.png)