How to set active buttons on the navbar

[mainframe]

I'm currently reverse-engineering the blogs and groups to see how they set the active tab button on the navbar. From what I can tell they use the node id but how could I do that with custom pages?

Edit: seems on nginx servers you need to replace {vb:raw _SERVER.REQUEST_URI} in the header template with {vb:raw page.url} to make tabs activate on custom pages.