This one is a bit alluding because it looks from the css that the body#bg class is driving the color. But in fact it’s a gif file called from the template_css.css file at line 584 and again at line 843 for the two sizes this template uses (narrow and wide).
Line 584:
.wide #ja-main {
background: url(../images/main-wide-bg.gif) repeat-y right #FFFFFF;
}
Line 843:
#ja-main {
border-top: 1px solid #FFFFFF;
background: url(../images/main-bg.gif) repeat-y right #FFFFFF;
}
Trouble is that the graphic also covers the right column so removing the reference to the graphic will make the middle and right background white. You might want to modify the gif so the left side of the graphic is white and leave the right side grey.