Highlight Parent page on Sub-Pages

I have sub-pages for a page called weddings (found here), and want to keep weddings highlighted even when I’m on it’s sub pages.
I’m using wp_list_pages to get my pages, so their coming out in lists automatically. My guess is a simple bit of PHP is all I need, but my php skills are pretty bad. I’m supposing the php would create a CSS class for the parent page’s li, and then I make some CSS happiness.

Any help is greatly appreciated.