From what I've read about them, they're an overlay and, from what I
understand, will manifest on native gnome application you install,
irrespective of whether it's come from Canonical or not (although, as
you point out, not necessarily everything). This suggests, to me at
least, that they should be switchable on or off somewhere - hopefully
someone out there has an answer.

I used to use KDE 3 but, with the advent of KDE 4, I do not get on
with it at all. Didn't click with xfce, either, sadly.