Actually, we have that already.
It's a defcustom called custom-theme-directory. It only holds one
directory, and it is safe to be customised by the user.

On 8 Oct 2015 4:46 am, "John Wiegley" <johnw@newartisans.com> wrote:
>
> >>>>> Artur Malabarba <bruce.connor.am@gmail.com> writes:
>
> > I'd like to downgrade this defcustom into a defvar, so that users cannot
> > customize it by accident. It should not be a defcustom in the same way
that
> > load-path isn't.
>
> > Would anyone object?
>
> What about having two load-paths, the way that we have
> Info-default-directory-list, and Info-directory-list. The former is
intended
> to be customized and is not added to, while the second is derived from the
> first initially, but possibly added to as Emacs runs.
>
> John
>