add background color on suggested topics list header

Before:

![screen shot 2013-06-20 at 10 58 26 pm](https://f.cloud.github.com/assets/362783/684106/a4af8aaa-d9ef-11e2-8149-6ec1a7d96e73.png)

After:

![screen shot 2013-06-20 at 11 23 33 pm](https://f.cloud.github.com/assets/362783/684109/adbdd08e-d9ef-11e2-828b-e528bfb92467.png)