bug#65145: ‘enable-theme’ completion for themes loaded by ‘require-theme’

[Shynur Xie]

> I can't reproduce it.  I did:
> emacs -Q
> (require-theme 'leuven-theme)
> M-x enable-theme

Oh Oh, sorry, Mauro.  I forgot to say that, this occurs only when
using a third-party theme, e.g., you create a file named ‘X-theme.el’,
which is under ‘/tmp/’ and contains the following code:

    (deftheme X)
    (provide 'X-theme)

Then evaluate

    (let ((custom-theme-load-path '("/tmp/")))
      (require-theme 'X-theme))

Then call ‘enable-theme’ interactively.

> It should offer the completion, and at least it does for me. What
> were the steps that you followed? And on which Emacs version?

- emacs-version: 29.1;    system-configuration: x86_64-w64-mingw32
- emacs-version: 30.0.50; system-configuration: x86_64-pc-linux-gnu