static <T> T getStyleFromToolBarUI( Component c, Function<FlatToolBarUI, T> f ) {
		if( c instanceof JToolBar ) {
			ToolBarUI ui = ((JToolBar)c).getUI();
			if( ui instanceof FlatToolBarUI )
				return f.apply( (FlatToolBarUI) ui );
		}
		return null;
	}
Because this border is shared for all toolbars,
get border specific style from FlatToolBarUI.
flatlaf-core/src/main/java/com/formdev/flatlaf/ui/FlatToolBarBorder.java