Sets the #GtkFrame:shadow-type for @frame, i.e. whether it is drawn without
(%GTK_SHADOW_NONE) or with (other values) a visible border. Values other than
%GTK_SHADOW_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_SHADOW_NONE) or with (other values) a visible border. Values other than %GTK_SHADOW_NONE are treated identically by GtkFrame. The chosen type is applied by removing or adding the .flat class to the CSS node named border.