outline-minor-faces background

modus-themes adds support for package outline-minor-faces by setting a background for face outline-minor-0, following in that regard the procedure of the package itself, that does the same, and this background is inherited by all other outline-minor faces. I suggest modus-themes perhaps should refrain from setting this background, for consistency. outline-minor-faces is just another instance of "outline headings", there included Org, of course, but also the built-in outline.el including outline-mode and outline-minor-mode (which recently had built-in font-lock support added for Emacs 28, in this being able to replace the package in question).

But the outline-N faces and org-level-N faces do not receive such background, being consistently set by modus-themes-heading-N. So, why should the outline-minor-N get the background if they are meant for the same purpose?

It is only fair if the package author likes it, or thinks this is the best way to display the outline headings for the user. But, since modus-themes has a pretty solid set of options for users to control how outline headings are presented, and present them consistently in all other cases, why not have outline-minor-faces also benefit from this infrastructure, and set it up consistently with other outline headings the theme offers?

I believe the face still has to be actively specified by the theme, since otherwise the default background will come through (or set all outline-minor-N faces). I do this by setting the :background of outline-minor-0 to 'unspecified, I'm not sure if the theme can handle this some other way.