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

[Drew Adams]

> retitle 15687 Disabling custom theme does not reset vars
> thanks

Oh, and this should really not have been so retitled.  Please see
the bug report.  This is about restoring the configuration before
switching to a theme.  It is not just about variables.

(Probably "initial configuration" in the original title is
misleading.  I meant initial in the sense of before applying
a theme, not in the sense of initial Emacs state: emacs -Q.)