Gtk.Container.prototype.set_border_width

function set_border_width(border_width:Number):void {
    // Gjs wrapper for gtk_container_set_border_width()
}

Sets the border width of the container.

The border width of a container is the amount of space to leave around the outside of the container. The only exception to this is Gtk.Window; because toplevel windows can't leave space outside, they leave the space inside. The border is added on all sides of the container. To add space to only one side, one approach is to create a Gtk.Alignment widget, call Gtk.set_size_request to give it a size, and place it on the side of the container as a spacer.