Okay, because I'm a perfectionist, I actually just posted an entry, and it's not perfectly solved yet. Your diagonal side strip will only show up to a certain point using the css from Minty Apple. Once you've done everything in that first comment, go back to the custom css area and look right at the top where it says *---- BODY----*

Find this bit of the source:

html {background: #333333 url( http://i45.tinypic.com/10emyaw.png) repeat-x 0 100px;}
body {font: 13px arial; color: #606060; margin: 0; background: url(http://i48.tinypic.com/2nkro29.png) repeat-y center !important; margin: 0 auto; }
#hello-world {clear: both;}

and replace it with this:

html {background: #333333 url( http://i48.tinypic.com/2nkro29.png) repeat-y center !important; margin: 0 auto; }
body {font: 13px arial; color: #606060; margin: 0; background: url(http://i45.tinypic.com/10emyaw.png) repeat-x 0 100px; height: 400px;}
#hello-world {clear: both;}

and everything should show up exactly like the original. :D (The more I do this, the more I think I really should get around to design as a career. I'm starting to scare myself with how easily I can manipulate code I've never seen before.)