bug#48736: 28.0.50; Regression in `enable-theme'

[Daniel Mendler]

On Emacs 27 the following snippet works as expected:

~~~
(defun set-theme (theme)
  (mapc #'disable-theme custom-enabled-themes)
  (when theme
    (if (custom-theme-p theme)
        (enable-theme theme)
      (load-theme theme :no-confirm))))

(set-theme 'deeper-blue)
(set-theme 'leuven)
(set-theme 'deeper-blue)
~~~

On Emacs 28, the second call `(set-theme 'deeper-blue)` is ineffective
and `deeper-blue` is not enabled. In contrast, the `set-theme` function
works, if written as follows, indicating a regression in `enable-theme`.

~~~
(defun set-theme (theme)
  (mapc #'disable-theme custom-enabled-themes)
  (when theme
    (load-theme theme :no-confirm)))
~~~

In GNU Emacs 28.0.50 (build 12, x86_64-pc-linux-gnu, GTK+ Version
3.24.5, cairo version 1.16.0)
 of 2021-05-23 built on projects
Repository revision: 04e7c6da34df6b60e253a35b9baa3eba4062617f
Repository branch: icomplete-affixate
Windowing system distributor 'The X.Org Foundation', version 11.0.12004000
System Description: Debian GNU/Linux 10 (buster)