static <T> T getStyleFromTreeUI( Component c, Function<FlatTreeUI, T> f ) {
		JTree tree = (c instanceof JTree)
			? (JTree) c
			: (JTree) SwingUtilities.getAncestorOfClass( JTree.class, c );
		if( tree != null ) {
			TreeUI ui = tree.getUI();
			if( ui instanceof FlatTreeUI )
				return f.apply( (FlatTreeUI) ui );
		}
		return null;
	}
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