GNOME 2 had it 15 years ago?
I guess even further, darn near every toolkit in the history of Linux/BSD GUI toolkits, except I suppose some embedded or hyper-minimal ones, supported custom themes that implement system-wide dark mode, and only relatively recently did GNOME and (some of) community decide app developers’ design choices were more important than user choice (eg. https://stopthemingmy.app/).
Just about any QT, FLTK, or GTK < v3 application already supports dark mode; that this enhancement (which takes another hard dependency on dbus, at that) is noteworthy is a bit unfortunate IMO. Their project, their rules, though, I have no ground to stand on here.