How would I achieve different styles for say, every third page?
Like the following CSS pseudo-selector, but for pages:

ul li:nth-child(3n+3) {  
  color: #ccc;
}

I have tried @page:nth(3), but that selects only the third page, not every third page.

We don't support this at the moment. You can access every second page, using the :left and :right page selectors for duplex printing. Styling every third page is a rather unusual thing to do.

Ok, thanks for the info.