RE: Why is custom--inhibit-theme-enable not t by default?

[Drew Adams]

> > AFAICS, you addressed the question of whether loading a theme
> > should activate it, but you did not address the point to which
> > you replied: disabling a custom theme does not undo it.
> 
> AFAIK M-x disable-theme RET should do the trick.

No, it doesn't.  It does not restore non-theme customizations
that were in effect before you enabled the theme.

Theme "disabling" is only relative to other themes - it does
not undo the changes made by enabling the theme.