newstyle :one of Gtk::RELIEF_NONE, Gtk::RELIEF_NORMAL or Gtk::RELIEF_HALF
The set_relief() method sets the relief style of the edges of the button. Three styles exist, Gtk::RELIEF_NORMAL, Gtk::RELIEF_HALF, Gtk::RELIEF_NONE. The default style is, as one can guess, Gtk::RELIEF_NORMAL.
See also: get_relief()