This might be relevant:
In 2019 at Guadec there was a meeting between interested parties with regards to theming.[0] This was the meeting where the current direction of Gnome theming was decided on[1][2]. System76 was obviously (see[0]) a participating party at that meeting.
At that meeting it was decided that everyone at the meeting should contribute to the theming api. System76 at the time announced that they
"First, we’ll work together to identify and document colors that can be exported and modified by theme developers. These include header bars, buttons, switches and other widgets that theme developers often modify. This work also enables app developers to more easily customize the look and feel of their apps."
Elementary and the Gnome developers took over the Dark mode api. Which has been implemented and delivered. It would have been Budgie, Ubuntu and System76's job to get together, write a specification of what they need and then work on a proposal of how they want to achieve that. They didn't do that in the last two years and now they are complaining that Gnome didn't do the job they (Budgie, System76, Ubuntu) agreed to do.
I know, hating on Gnome is easy sometimes. But that really doesn't look like the Gnome devs are at fault. They communicated their intents and reasons years and years ago[3]. This should not been a surprise and complaining loudly about this now - to say the last - a bit dishonest with regards to their initial involvement.
There's still work with regards to a theming API by Purism's Alexander Mikhayelenko, but that work is ongoing[4].
[0] https://blog.system76.com/post/187541504063/the-future-of-th...
[1] https://discourse.gnome.org/t/gtk-adwaita-and-vendor-styles/...
[2] https://etherpad.gnome.org/p/VendorThemes2019/timeslider#161...
[3] https://stopthemingmy.app/
[4] https://gitlab.gnome.org/GNOME/libadwaita/-/issues/215