Libadwaita hasn't crippled that, it's still very possible to load CSS. You just can't use the gtk-theme-name setting anymore.
Maybe it would be better if somebody fixed that thing with the menus. But I think IIRC if you wanted transparency in old X11 without a compositor, the toolkit had to copy the region under the popup, and then blit it to the popup... Yes, this meant that content in the transparent region that changed would be stale until the popup went away. So someone would have to decide if that would be a viable solution, or maybe someone could get away with just using the existing XShape stuff in GDK somehow.
Maybe it would be better if somebody fixed that thing with the menus. But I think IIRC if you wanted transparency in old X11 without a compositor, the toolkit had to copy the region under the popup, and then blit it to the popup... Yes, this meant that content in the transparent region that changed would be stale until the popup went away. So someone would have to decide if that would be a viable solution, or maybe someone could get away with just using the existing XShape stuff in GDK somehow.