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

How can it be both a marathon and a gamble?

Seems like you have a permissions issue. I just tested it on Fedora workstation gnome, run it from regular menu, then it asked me for password. You, you have a permission denied isssue, so you need to figure out how to run it as root.