This guide/video is out of date and should be recreated - Pete 01/09/2023 (Some extensions no longer work and have been changed to something else, and I use a different clock now, however the main overall processes are still valid so leaving up).
Theming is pretty long winded so I made a video guide. I don't use Ubuntu much so apologies for the confusion in places. Ubuntu is very good just has some quirks:
Example of themed Debian:
Just a note on Extensions: Often as gnome updates (eg: from 41 to 42) all the gnome extensions you have stop working. If you theme heavily you might find extensions being turned off and you have to wait until they get updated before they work again. Just a risk when running latest of all your software. So if that's an issue it might be easier to just use gnome in its vanilla style.