static <T> T getStyleFromListUI( Component c, Function<FlatListUI, T> f ) {
		JList<?> list = (JList<?>) SwingUtilities.getAncestorOfClass( JList.class, c );
		if( list != null ) {
			ListUI ui = list.getUI();
			if( ui instanceof FlatListUI )
				return f.apply( (FlatListUI) ui );
		}
		return null;
	}
Because this border is always shared for all lists,
get border specific style from FlatListUI.
flatlaf-core/src/main/java/com/formdev/flatlaf/ui/FlatListCellBorder.java