Comment 4 for bug 284975

The issue at hand, as I understand it, is that Ubuntu sets a background image for the panel /outside/ of the theme. If you take a look at Panel Properties, you'll notice that the background tab has (instead of "None. Use system theme"), an arbitrary background image set by default.
This will cause confusion for users and issues with, for example, low contrast themes.

I believe this is a hack to fix a bizarre issue where some applets seem to cope fine with drawing themselves on top of a user-specified image background but fail miserably with all sorts of awful transparency problems when they are drawn on even that same background as part of the theme. However, the fix is problematic for the reasons outlined here...
Personally, I still think it's worthwhile just for the sake of having a nice looking panel, but perhaps a few little hacks will solve it completely. (For now).

Deepest apologies if I have managed to double post this.