Further investigation reveals that I can call setBackground(null) to get
the Composite back into the "default" state.
This will probably work for my immediate need; however, I don't see a way
to tell if the Composite was in the "default" state before the first
setBackground() (in other words, I can't handle the case where the
background was previously set to a non-default state, because I can't ask
the composite if it has been set or not.)
Question: If this were to be added (the ability to tell if a control's
background is in the default state), would it be more appropriate to have
"getBackground()" return "null", or would it be better to add an
"hasDefaultBackground()" method?