Background under avatar column

I was wanting to change the background of the column area where the avatars are located, like on this forum its on the right. Our avatars/column is on the left with the modules on the right. But I wanted to change the background of the left column where the avatars are located. I wanted to know where to add the image background CSS code into the stylesheet so it will show in the avatar column on the forum.


In other words, what part of the CSS style sheet addresses the background under under the avatar column.

Thank you