For anyone else landing here from #1722/#1774 (or #1659/#1757): notarized builds of Skim with these available as options live at https://github.com/speckofthecosmos/skim-plus — an Advanced preferences pane with the inverted page background level (down to true black), background colors that render as picked under the invert filter, tooltip timing, and a View-menu toggle for invert. The patches only expose options: with nothing set, the builds behave exactly like stock Skim, and new upstream releases...
Apologies — this duplicates #1774, which I missed before posting. I see from there and #1722 that the gray level is deliberate, so no need to relitigate. I'm running a locally patched build (f = 2.0) which covers my OLED/outdoor case.
True black inverted pdf color