IIRC the debacle about theming was:

a. Only about programs using libadwaita b. About their opinion that just overriding the global style like in GTK3 was causing too many issues in apps defining their own widgets or CSS to be worth it.

IIRC they were willing to accept a contribution of a more advanced theming system (but building it themselves was not something they wanted to prioritise over other things), but lacking that they’d rather enforce using adwaita in libadwaita.