viceng is right. You need to edit style.css (line 168)
#mainpage {
background-color:#FFFFCC;
background-image:url(https://www.sketchygrid.com/images/bg_2.png);
background-position:right top;
background-repeat:repeat-y;
border-right:1px solid #000000;
min-height:1000px;
padding:0 10px 10px;
width:100%;
}
and change background-repeat:no-repeat; to background-repeat:repeat-y;
I tried that originally and thought it wasn’t working. However, I’ve just pulled down a copy of the background image and I can see why. It’s an 800px high image – most of which is actually plain yellow. Not sure you’ll be happy with the result when you do get it tiling down the page.