I have 2 desktops, GNOME and MATE.
As far as I know the GNOME 3 theme lives under [~/.config/gtk-3.0
On right-click > Change Desktop background > Appearances and Themes > It's menta with some customizations which is part of mate-themes and it's at 3.20.7-1 on Debian unstable/sid. This shows Menta look with some modifications. Menta is both a 2.0 and 3.0 look/theme.
[/usr/share/themes/Menta] └─[$] ls cinnamon emerald gtk-2.0 gtk-3.0 index.theme metacity-1 README unity xfwm4 [~/.config/gtk-3.0] [$] cat settings.ini [Settings] gtk-theme-name=AlaDelta gtk-icon-theme-name=matefaenzagray gtk-font-name=Sans 12 gtk-cursor-theme-name=DMZ-Black gtk-cursor-theme-size=48 gtk-toolbar-style=GTK_TOOLBAR_BOTH_HORIZ gtk-toolbar-icon-size=GTK_ICON_SIZE_LARGE_TOOLBAR gtk-button-images=1 gtk-menu-images=1 gtk-enable-event-sounds=0 gtk-enable-input-feedback-sounds=0 gtk-xft-antialias=1 gtk-xft-hinting=1 gtk-xft-hintstyle=hintmedium gtk-xft-rgba=none
Look forward to know which theme am I using.