Themeable SectionBg

Allows the styling of SectionBg() to be controlled by the gtk theme
![section-1](https://cloud.githubusercontent.com/assets/2231759/2534396/a246f4f8-b57a-11e3-9a8c-25dcba8f7d43.png)
![section-2](https://cloud.githubusercontent.com/assets/2231759/2534399/ab58f7bc-b57a-11e3-96af-cc9a5e110ef5.png)