Newbie questions on themes and engines

["Eckhoff, Michael A" <EckhoffM NAVO NAVY MIL>]

1. Is there a discussion somewhere on themes and engines? Do these terms
refer to 
the same thing?

2. Who sets the default theme for gtk+ applications? Does this default come
with
the package or does my Linux vendor (Suse in my case) have anything to do
with it?

3. What other themes are available, and how do you invoke them?
e.g., as a command-line option like ./helloworld --alternate_theme

Simple questions, but I can't find the answers in the usual places (unless
I've overlooked something).

Thanks,
Mike