How to tweak public navigation markup?

I'm writing an Omeka custom theme an I need to mark the currently active tab in navigation. This seemed simple task to me, since the current tab has <li class="active"> in the code generated by the public_nav_main().
The problem is, that I'd like to have the "Items Browse" tab active for /items/show/# and /files/show/# pages. Are there any parameters to be passed to the public_nav_main() function (or any other non-javascript method) to achieve this?