Hi, you may want to check my comment here first to see if that would work for you: Home directory variable in gtk.css - #3 by jfrancis
The problem I think with loading an adwaita.css and such is that Adwaita is actually four themes: light, dark, hc-light, hc-dark. So such a scheme would likely have to take that into account. Maybe at some point media queries could be added that would work for these? Eventually though it seems something like that would be needed, because anything in gtk.css is also going to interfere with other apps that don’t use adwaita too.