But ultimately the beauty of Andrea’s approach is that we can just put
random things in there, write EXPERIMENTAL in all caps in the docstring,
and refine until 28. What we initially put in there is not all that
important. to be honest.
I don't agree with this. We should not throw "random things" in there
because that will affect the possibility that Andrea's customization
can be composed with Joe Doe's, whose contribution I would also
welcome. If we follow your plan, it seems very likely that a sub-battle
for defaults emerges within it. So, no to "random". Only things that
our "custom-theme" infrastructure knows how to handle. If it doesn't
handle some of those, then either that infrastructure or a package's
custom.el conformance should be augmented.