stocc Gnome Software uses gtk4+libadwaita and they don't really support theming. (If anyone want to discuss this decision, please create another thread). Only light and dark mode are available.
It is possible to workaround it but not very easy. You need to edit the gtk4 css file/directory manually. Some themes have the instructions for it on their repo. Or you can use adw-gtk3-theme
to make other (gtk) apps looks like gtk4+libadwaita. You probably already have a preferred theme.