/etc/gtk-3.0/settings.ini is in libgtk-3-0 3.18.9-1ubuntu3.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
1 2 3 4 5 | [Settings]
gtk-theme-name = Ambiance
gtk-icon-theme-name = ubuntu-mono-dark
gtk-sound-theme-name = ubuntu
gtk-icon-sizes = panel-menu-bar=24,24
|