changing page id's

[Dave Egerton]

Can anyone explain how I can change the page id, depending where I am in Prestashop.

On the demo, in categories, the page id=categories, in products, the page id=products

I can see this code (in themes)
header.tpl
body {if $page_name}id="{$page_name} or
div id="page"

But l cannot see how its being applied to pages and how I add new ID's

is taht possible?