A while back there was a lot of talk about Canonical implementing subtle transparency in GTK for Ambiance and Radiance themes – Is this no longer on the agenda? Was there a hold up?
Ubuntu – Are there still plans to implement transparency (RGBA) in Ubuntu’s theme
gtkthemes
Best Answer
The problem with implementing rgba currently is lying with the applications that run on the gtk+ library. However, with the switch to gtk+ 3 it is likely that Ubuntu will inherit this feature as a default option.
I'm sorry I don't have a link to hand, but the feature was not dropped, just hindered.
EDIT: Here is a link that indicates the possibility of rgba as a feature of gtk3 (which Ubuntu uses as of 11.10).