To show you the CSS, assuming your background file is called "bg.jpg" and is in the root folder next to index.html, look for the "body" in a file called "custom.css" and add these bits:
body {
background-color: #666666 !important;
background-image: url(../bg.jpg);
background-repeat:repeat;
}
Also look at that tutorial for other options for background-repeat.