bug#65401: 30.0.50; custom-theme-directory :type could be enhanced

[Eli Zaretskii]

> Date: Sun, 20 Aug 2023 08:28:48 -0300
> Cc: 65401@debbugs.gnu.org
> From: Mauro Aranda <maurooaranda@gmail.com>
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
>  > Does the directory have to exist?
> 
> I don't think so.  And I don't see a reason to force the existence by
> putting a non-nil :must-match property.
> 
> The function custom-theme--load-path takes care of filtering out
> custom-theme-directory if it doesn't exist.

If the directory doesn't have to exist, then what is the difference
between :directory and :string?