As I struggle my way through the configuration files, GTK+ documentation and some source code, the Ubuntu Theme takes on shape.
I still have not found out how to distinguish between windows in the front (focused for Metacity) and the ones in the background (normal). Having a way to differentiate would allow for different coloring of widgets. This in turn would make for nice visual feedback.
Another issue I have to tackle with is figuring out the names of widgets in different applications. Not all of them are documented in the GTK documentation.
Finally my icons in the top panel grew all of a sudden from nice 28 px to 32px filling the whole panel. I have no clue what I did (acutally I think I did nothing, but that certainly is not the case). I would like to think this is due to errors in all the other themes.