These days, after bug 1853323, about:PreferenceStyleSheet is basically
just an empty sheet for most users. For the ones that isn't, it's either
slightly or completely broken (see below).
If we were to implement this today, we'd do it with privileged env()
variables or media queries, and add a bunch of tests for different
combinations, but I don't think it's worth the effort:
* There's no UI for any of these prefs.
* There are no tests for any of these prefs.
* browser.display.focus_ring_or_anything is not great, it's better
replaced by browser.display.show_focus_rings, which actually makes
the :focus-visible pseudo-class work. Otherwise the native controls
may show no rings even though this pref is set.
* for browser.display.focus_ring_style = 0, the color used
(-moz-mac-focusring) is not implemented in any non-macOS platform.
* The ::-moz-focus-inner rules are completely useless nowadays, because
::-moz-focus-inner only works on the windows native theme (which
isn't used on content).
* The focus colors are not color-scheme aware.
So given those, and that if you want the specific current behavior,
these are trivially implementable as a web extension (by using a user
stylesheet), I think we should just remove this code altogether.
Differential Revision: https://phabricator.services.mozilla.com/D190464
56346e9adf