Background.

So, just look for these codes

body {
background:$bgcolor;
margin:0;
color:$textcolor;
font: x-small "Trebuchet MS", Trebuchet, Verdana, Sans-serif;
font-size/* */:/**/small;
font-size: /**/small;
text-align: center;
}

Then, instead of ... change it to


body {
background-image: url(here should be your background URL); background-position: bottom right; background-repeat: no-repeat; background-attachment: fixed;
background-color:$bgcolor;
color:$textcolor;
font: x-small "Trebuchet MS", Trebuchet, Verdana, Sans-serif;
font-size/* */:/**/small;
font-size: /**/small;
text-align: center; }

Notice the change? Some codes were added. :)
Now, you have a background image. :)