~/.config/gtk-3.0/settings
This colorized verison of my GTK3 configuration file is available for your convience.
The original file is available for download if you would like to use it.
[Settings]
gtk-font-name=Sans Serif 9
gtk-theme-name=oxygen-gtk
gtk-icon-theme-name=oxygen
gtk-fallback-icon-theme=elementary
gtk-toolbar-style=GTK_TOOLBAR_BOTH_HORIZ
gtk-menu-images=1
gtk-button-images=1