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

[Eli Zaretskii]

> Date: Sun, 20 Aug 2023 07:48:09 -0300
> From: Mauro Aranda <maurooaranda@gmail.com>
> 
> The user option custom-theme-directory is supposed to hold a directory
> name.  But its :type is a string.
> 
> I realize that it isn't really a bug, but in order to offer better
> completion, at least, I'd like to change the type to be directory.  So
> I'm filing this bug report to see if people would object to such a
> change.

Does the directory have to exist?