There are a few different GTK theme configuration tools out there – lxappearance (which doesn’t require the LXDE desktop environment), gnome-tweaks, and xfce4-appearance-settings – to name a few, as well as some desktop-environment agnostic ones such as gtk-chtheme (which supports GTK2 only).
However, my current favorite tool which works for GTK2, GTK3, and GTK4 is nwg-look:
