Support dark variation of the theme on a per-application basis

gtk+ ≥ 3.12 supports loading different themes and dark variants[1]. I just upgraded a VM to the latest state of artful the check the status of the Gnome transition and the default Ubuntu theme (Ambiance) does not seem to support this at the moment.

Some applications like the lollypop music player make use of this[2]. Adwaita as well as other themes like Arc have support for a dark variant (not to be confused with Adwaita Dark or Arc Dark). Here is an older example on AskUbuntu with screen shots for illustration[3].

Can someone give a rather experienced Ubuntu user like me (with no actual contributions so far) a heads up about what would be needed to get involved and help out to implement this.

Related older bug reports:

LP: #903833 Support dark variation of the theme
LP: #158392 Ubuntu need a dark theme and background

1: https://unix.stackexchange.com/a/185115/49853
2: https://github.com/gnumdk/lollypop/issues/467
3: https://askubuntu.com/q/806200/40581