If you look at layout.css from the modxhost page, you’ll see that this grey is being created with an image:
#outer{
...
background: url(images/inner_776x1.jpg) repeat-y left top;
}
That jpg is 776 pixels wide and only one pixel tall and stretched vertically. It includes the off-white left background as well as the grey right sidebar, the line betwen them, and the vertical borders. This is a pretty common trick when creating pure CSS sites, because otherwise it can be difficult to get your various column backgrounds to always stretch to the same height.
So it depends on what kind of image you want to put in the right sidebar... You can overlay an image on top of this one (or another one with colors of your choosing) if you add it to the right or right-inner or sidebar style. But there are some visual effects that you may not be able to get without changing the overall layout of this template, since all two-column layouts are not created equal.
I suggest that if you can’t get exactly the effect that you’re looking for by modifying this template that you do a Google search for CSS two-column (and three-column) layout examples. You’ll find tons, some of which may be exactly what you’re looking for. Also check out www.oswd.org and other collaborative web design sites.