Sets the #GtkFrame:shadow-type for frame, i.e. whether it is drawn without
(gtk.types.ShadowType.None) or with (other values) a visible border. Values other than
gtk.types.ShadowType.None are treated identically by GtkFrame. The chosen type is
applied by removing or adding the .flat class to the CSS node named border.
Sets the #GtkFrame:shadow-type for frame, i.e. whether it is drawn without (gtk.types.ShadowType.None) or with (other values) a visible border. Values other than gtk.types.ShadowType.None are treated identically by GtkFrame. The chosen type is applied by removing or adding the .flat class to the CSS node named border.