Now that GTK port no longer uses foreign drawing to render widgets, since r257299, it's finally possible to support the color-schemes CSS property. (See bug #197947 for discussion of why this was not previously possible.)
Overview of how dark mode works currently:
Firefox and Chrome and WebKitGTK since r255342: indicate to the webpage whether user prefers a dark theme using prefers-color-scheme, but always render controls using light theme. (GOOD)
Safari only: indicates to the webpage whether user prefers a dark theme using prefers-color-scheme, renders light controls usually EXCEPT if website opts-in to dark form controls using color-schemes CSS property. (BEST)
Downsides: this would complicate the new built-in Adwaita theme, so we might not want to do it.
Well, I think we can try, but I want to give some more time to the new theme code, get feedback and improve it a bit before adding a new feature like this. We have the whole release cycle :-)