Personally I believe CSS to be quite ill-suited for the purpose. It’s ok if you’re writing a theme for a bespoke one-off app but breaks down in the system theme use case. In particular, CSS inheritance makes for a lot of unnecessary trouble for both third-party themes and accessibility affordances.
Last I knew there was something of a disinclination away from paramaterization in the GTK dev sphere too, which is another significant problem for third party themes and accessibility. Hardcoded fonts, colors, etc makes for pointless brittle rigidity.