Selected item in navlinks

I was wondering about this too There is class="isactive" but its only used on Profile and Admin menus.

Could this be added to the navigation links in the core also? Good for usability I think

It looks to me on that setup that index is only active if you are on the index page.  Shouldn't index be the active link for any page that descends from it and doesn't have its own menu item, basically any page with breadcrumbs.  If not then if you are on viewforum or viewtopic there is no active menu item.  Either there is always an active item or never an active item.

An alternative way of doing it would be
define('FORUM_ACTIVE_PAGE', 'whatever')
And then used the same value on descendant pages e.g. index, viewforum and viewtopic would all have been 'index' and all profile pages would simply be 'profile' as would sub-forms such as change password, avatar upload etc.

Why did you use FORUM prefix instead of PUN?