From http://wiki.mdz.com.au/index.php?title=Homepage_Specific_Class
Adding body/class tags for various dispatches
<body id="{$runtime.controller}" class="{$runtime.mode}{if $runtime.controller=='categories'} category{$smarty.request.category_id}{/if}{if $runtime.controller=='pages'} page{$smarty.request.page_id}{/if}">