Is it possible to base a custom GTK+ theme on Adwaita without having to compile a custom GTK+?

I planned on following https://worldofgnome.org/creating-g... to solve a scrollbar styling issue for myself.
But that guide seems to be horribly outdated as Adwaita seems to be fully compiled in as an engine now - for performance reasons, wich they obviously would never expect to matter to people using custom themes (because else they would have optimized the application of all themes in general instead)...