by default GNOME is using libadwaita traditional way to use themes like in gtk before is not working anymore… you can still apply theming changes but needs something like this:
There are some extensions that can help color scheming
Gradience
but still partly only an idea… if you are using apps based on gtk2 or 3 … they will not follow … qt app? will not follow…
“legacy” apps will need to set theme in tweaks tool where it takes themes you saved in theme folder at users home …