You can use a black theme. Look at gnome-look.org

You can also reverse engineer them to look how they make the theme blank and apply it to adwaita. This is not complicated as themes are mostly CSS.

Edit to add that you can chat with people from open deskttop (gnome look) to ask your question: https://chat.opendesktop.org/#/welcome

libadwaita-without-adwaita and use a theme.

And, i use Kvantum for Qt apps on XFCE but think it works the reverse too?