bug#15687: 24.3.50; custom themes: disabling does not restore initial configuration

[Drew Adams]

> I give up.  If you want to remain convinced there is no problem,
> fine.  If you don't want to even try to see the problems reported,
> using the simple recipe I gave, fine.  (Yes, simple to do: download
> the files, load them into emacs -Q, and try the command.  Maybe 3
> minutes altogether, including the time to download.) ... there
> seems to be little desire to fix the custom theme code in this
> regard.  So be it.
>
> Ok; closing.

There is nothing OK about closing this bug.  The bug clearly
remains, and it is 100% reproducible.

And users of custom themes keep getting tripped up by this
problem.  A common, perhaps the most common, question about
custom themes is how to completely undo one.  IOW, this bug.