Permanent Webpage Background in CSS

Permanent Webpage Background in CSS

I currently am working on a basic website for a friend who wants a simple
blog/website, and have a quick CSS question.
http://nashread.com/template1 Is the current domain, and if the background
on there is fine and covers the whole screen. That's because the content
box isn't long enough to extend the page. If you click on the 'blogs' tab,
you'll see that if the content box extends the background, it just goes
into white space. Basically what I'm asking, is how would I make it so
when you scroll it only scrolls the content box, rather than the whole
webpage? It might not even be possible in HTML or CSS, so if so please
tell me.
Thanks (=