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

[Drew Adams]

This bug should not have been renamed from "custom themes: disabling does not 
restore initial configuration" to "disabling custom theme does not reset vars".

It is *not about resetting variables*.  Not at all.

It is about restoring any number of settings that might have been changed 
(including customized using Customize) prior to enabling the custom theme: 
faces, frame parameters, etc. - as explained in this bug thread.

The point is that "disabling" a custom theme does not restore the pre-enabled 
state of everything that the theme changed.  Disabling only restores things 
relative to another custom theme.