Sets the 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 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.