Add gtk-dialog-color choice in awn-prefence

Sometime with many deficient gtk theme the gtk-dialog (like "bubble" for group common windows) in awn are unreadable.
For an example you can see http://pix.toile-libre.org/?img=1268669879.png.
It's not a awn's bug because if I disable dialog_gtk_mode (in gconf-editor) , and change the dialog_bg key manually; it's readable.
http://pix.toile-libre.org/?img=1268752300.png

Unless, I think it would be more easy if this manipulation can be execute in awn-preference rather than in gconf.