The nav links’ CSS is the same on all pages, the problem is how to insert the appropriate class(es) in the body tag. That can preferably be done serverside.
Clientside you can skip this method and just use javascript to insert an active class in the menu/submenu according to the url of current page. That script can also delink the active nav (sub)link, that’s good.
With an sitename-class/id and/or page class(es) in the body tag, we hwo uses userContent.css for styling sites as we go would appriciate that.