Gives fieldsets a background color (nested fieldsets an opacity)

Before:
![screen shot 2014-11-18 at 5 16 51 pm](https://cloud.githubusercontent.com/assets/186715/5099236/ecb56322-6f46-11e4-825b-4857f91ab5a1.png)
![screen shot 2014-11-18 at 5 16 55 pm](https://cloud.githubusercontent.com/assets/186715/5099237/ef400d9a-6f46-11e4-8a9c-738d11cdab46.png)
![screen shot 2014-11-18 at 5 17 36 pm](https://cloud.githubusercontent.com/assets/186715/5099238/f2007510-6f46-11e4-9085-0d6a1d771f4c.png)

After:
![screen shot 2014-11-18 at 5 14 59 pm](https://cloud.githubusercontent.com/assets/186715/5099240/f5a8bac4-6f46-11e4-8c01-204fd242a4c2.png)
![screen shot 2014-11-18 at 5 15 05 pm](https://cloud.githubusercontent.com/assets/186715/5099241/f87fb450-6f46-11e4-8446-177871bcc5cb.png)
![screen shot 2014-11-18 at 5 15 13 pm](https://cloud.githubusercontent.com/assets/186715/5099242/fafc5472-6f46-11e4-9e11-a5f7fcbc5d39.png)