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)

After:
![screen shot 2014-11-19 at 9 18 01 am](https://cloud.githubusercontent.com/assets/186715/5110372/0a369ab6-6fcd-11e4-9b2e-2ad30cc661f8.png)

Before:
![screen shot 2014-11-18 at 5 16 55 pm](https://cloud.githubusercontent.com/assets/186715/5099237/ef400d9a-6f46-11e4-8a9c-738d11cdab46.png)

After:
![screen shot 2014-11-19 at 9 18 08 am](https://cloud.githubusercontent.com/assets/186715/5110373/0bbb2aaa-6fcd-11e4-8193-e1e1619550eb.png)

Before:
![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 2014-11-19 at 9 18 17 am](https://cloud.githubusercontent.com/assets/186715/5110374/0d559170-6fcd-11e4-943a-c0156e3d378c.png)