it is unlikely to be accepted if it makes the application override the theme.
It doesn't really _override_ the GTK+ theme, it just [requests the "dark" variant](https://developer.gnome.org/gtk3/3.2/GtkSettings.html#GtkSettings--gtk-appli...) of the same theme, if one exists. Given that many IDEs/editors these days come out of the box with a dark widget theme (ex. QtCreator, VisualStudio, Atom, VSCode, SublimeText, etc.) it seems like a good use case.
makes the point that there can be no objection to it being implemented as a plugin
Other than it being 60+ lines of boilerplate for 2 function calls :)
Anyway, I think we should leave this open for now since it's a highly requested feature than can be implemented without a lot of code changes if someone feels like doing it. I might have a try at doing it myself if I get bored. Even though you can technically get the same effect by fiddling with several different obscure config files and preferences, IMO it would be super convenient to just have a "dark mode" checkbox that satisfies people who have tired eyes.