CSS class for active nav link

You can already do that but it does require one style per link. The question as always is whether it is worth adding several lines of php to potentially save several lines of css. Personally, I'm in favour but that could be because my jobs writing the css not the php

You can because each page has its own id so you make descendant selectors

#punindex #navindex a:link, #punindex #navindex a:visited {highlight}
#punsearch #navsearch a:link, #punsearch #navsearch a:visited {highlight}

It works because the highlight style will only operate on, for example, the search link when the page has an id of punsearch.

In the case of index you would have to make a few extra styles because viewforum and viewtopic are really sub-pages of index eg. #punviewtopic #navindex etc.