GTK+ themes, I think the metacity approach to themes worked
out pretty well. You could definitely improve on the details
(inventing my own little expression language was kinda weird, and
there are lots of
feature requests for the format). But metacity themes are
“just data” rather than code while remaining reasonably flexible.
Arguably you want to go even more in the direction of “just data”
and keep themes to a pixmap-engine type of thing preferably with a GUI
theme editor. Lots of theme designers don’t want to mess with editing
an XML file anyway.
(This post was originally found at http://log.ometer.com/2005-11.html#26.2)