static void setStyleColorFromTreeUI( Component c, Graphics2D g, Function<FlatTreeUI, Color> f ) {
		Color color = getStyleFromTreeUI( c, f );
		if( color != null )
			g.setColor( color );
	}
Because this icon is always shared for all trees,
get icon specific style from FlatTreeUI.
flatlaf-core/src/main/java/com/formdev/flatlaf/icons/FlatTreeCollapsedIcon.java