Lightening up window decorations

Hi,
I don't see that there is a mailing list, wiki, or that sort of thing 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)