I have installed fedora 38 , I’ve tried many GTK themes ( Orchis , WhiteSur , Nordic ) , all Fatpak apps have those black bars around them. I have tried This solution , but it didn’t work.
Welcome to Fedora @james95373
Did you do a new login/reboot after the changes?
Did you check if this themes are all gtk-4.0 themes? If not, are this gtk packages in other versions available?
With the tweaks tool you can select the correct legacy-themes, if you use not using a pure gtk apps.