Stefan Monnier <monnier@iro.umontreal.ca> writes:

>>> Or if eventually a defface or a defcustom makes its way into
>>> display-fill-column-indicator.el, of course.
>
> Actually, there is a defcustom there (in the expansion of
> `define-globalized-minor-mode`).

Sorry, I wasn't clear enough.  There is no defcustom or defface whose
:group is display-fill-column-indicator.