Theme Engines

[kwall at kurtwerks.com]

Here's my question o' the day. Actually, it's more like a set of
related questions:

1) What are "theme engines", the obvious tautological answer
   notwithstanding?
2) What do they do?
3) Why would I choose one over the other? 
4) What are the relative merits and demerits of the theme 
   engines available?

Thanks,

Kurt
-- 
Putt's Law:
	Technology is dominated by two types of people:
		Those who understand what they do not manage.
		Those who manage what they do not understand.