Earlier quoted context omitted.
With a bit of effort it’s a solved problem, however as soon as that was sorted, developers started using snaps and flatpaks that just Do Not Care about your system settings... and we’re back at square -1, because now you won’t even get consistency between Gtk apps and Gtk apps. The Linux desktop sometimes is like a bad comedy sketch.
Do snaps/flatpaks really not look at your local configs? Thats... terrible. I've avoided them already anyway because they seem like wholly unnecessary (and quite significant) bloat.
While the most popular themes are available, more "niece" themes are missing. Do a `flatpak remote-ls flathub | grep --color "org.gtk.Gtk3theme"` to see all available themes.
Note: I'm maintaining a theme on flathub.