Sable is a multi-layered set and you're welcome to "view source" and copy the HTML code for the backgrounds setup if you'd like.

The code is already set in such a way as to give you nice even margins like this, and keep your content from running to the very edge of the page.

(*Unless your writing a poem...) do not 'center' your text!*) and you'll have a much nicer and more balanced looking page.