Seems to be caused by GtkWidget-wide-separators: true; in /usr/share/themes/Ambiance/gtk-3.0/gtk-widgets.css. This is not set in Radiance.
Seems to be caused by GtkWidget- wide-separators : true; in /usr/share/ themes/ Ambiance/ gtk-3.0/ gtk-widgets. css. This is not set in Radiance.