I don't know how well would that play on normal UI development. I think it would be a bit of an annoyance to need to always remember the dark mode.
My ideal solution would be to use CSS variables that a dark theme could override. Example:
I think it's the only way to make it so that the changes will scale. It still has coupling problems, as the CSS variables would then become an API that plugin stylesheets could use. It also needs some deep refactors and changes to happen, but no lesser than having to add a dark comment to all styleshets.