... cos I'm an idiot.

So, I'm just trying to make a simple template for a site I'm building for a charity. It just has a <header>, a <menubar>, and then a <main> content area with a <sidebar>. All this in a <wrapper> on top of a <background> div. Simple.

So why wont my chosen tone of silver-gray background color extend down the whole page?!! It only reaches down to the menubar and wont underlay the main content area although everything is inside the <background> div and it should stretch to fit, no?

I've hosted it here with all the content removed for ease of pointing out what fundamentally stupid mistake I'm making - please help!:

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