Lightening up bg_color

Hi,
I don't see that there is a mailing list for discussing the Greybird theme, so I'm just making my suggestions in the form of this Pull request. Feel free to reject it. All I really did was lighten the background slightly, giving Greybird a more polished feel comparable to, say, the default Gnome/Adwaitha or Elementary themes. Example screenshot of Thunar:

![thunar-screenshot](https://f.cloud.github.com/assets/1571667/304327/44959a04-963a-11e2-8d00-82282e1928d5.png)