Michael Albinus <michael.albinus@gmx.de> writes:

> I've added the test case custom--test-theme-variables in
> test/lisp/custom-resources/custom--test-theme.el for showing the effect.

The test is failing for a different reason.  With the call to
custom-set-variables, both variables will end up having a user theme
setting, and since the user theme has the highest priority, enabling
the custom--test theme makes no difference.  The following check fails,
because of the user theme having the highest priority:

;; The variables have the theme values.
(should (equal custom--test-user-option 'bar))
(should (equal custom--test-variable 'bar))

IOW, the variables have the user theme values, and not the custom--test
theme values.

Best regards,
Mauro.