Row in Wayfinder means each item. It may be <li>... </li>, or it could be whatever other elements you use in your templates.
What you would get would be
<li id="item1">....
<li id="item2">...
etc
if you specify $rowIdPrefix=`item`, and in your template had [+wf.id+], each row (a row being each link) would get an ID of "item" + its document ID. So if your Forum page is #7, its link would have an ID of item7. Then in your css, you would simply have #item7 styled with red.
If you only need this in the first level of the menu, you could use a template for "parents", and just have the [+wf.id+] placeholder in that template. Then only the top-level items would have the id. I’ve done this a few times in some of the sites I’ve built.