On a local installation, the XF navigation template was stripped out and the WordPress menu built by hand for a WP child theme was added. This works. Next, the style.css from the WP child theme and parent theme were added to the navigation.css. This works. Now - instead of copying the style.css into the navigation.css - Can the link command be used? In a few tests, just putting the <link .... > with the navigation.css area did not work. Maybe this should be wrapped with a <xen .... > ?