Existing implementations too! :)
I found the xfce gtk2 engine pretty simple and informative, although
Clearlooks didn't look too complex after that either...
I'm interested in the clearlooks one, because I interested in learning
how to use cairo to perform an engine, so I walked through the code but
I was unable to figure out the structure, so I was looking for more
"formal" documentation.
A reference implementation with all the autotools and scanner magic
done for you and stub draw functions which call the parent (ie.
default theme) would probably be cool to have. I just might create one
if I get the time...
Post it on the list if you do it :)