I'm using Ubuntu 23.04 for just a few days and tried to install user themes with the Gnome-tweaks tool. However the themes do only affect the tweaks app and the top bar, but not the File Manager or the Settings app.
Gnome transitioned to a new version of the graphical toolkit, GTK4. Some themes that were not updated may therefore not work for GTK4 applications. Both Files and Settings in Ubuntu 23.04 are GTK4 apps. Tweaks, in contrast, is still a GTK3 app.
The term "User themes" you used normally is used only for themes that are intended to change the appearance of Gnome Shell. Such themes will never affect the appearance of user applications. They only will change the appearance of the shell (black topbar and associated menus, overview, application overview). To install such "user themes", you need to have the "User themes" shell extension installed.
Gnome transitioned to a new version of the graphical toolkit, GTK4. Some themes that were not updated may therefore not work for GTK4 applications. Both Files and Settings in Ubuntu 23.04 are GTK4 apps. Tweaks, in contrast, is still a GTK3 app.
The term "User themes" you used normally is used only for themes that are intended to change the appearance of Gnome Shell. Such themes will never affect the appearance of user applications. They only will change the appearance of the shell (black topbar and associated menus, overview, application overview). To install such "user themes", you need to have the "User themes" shell extension installed.