I found the following code by inspecting the left column with Firebug, the "background" line is the color you want to change; it is located around line 415 of css/template.css in the theme folder:
#s5_split_layout_column_wrap_outer {
display: table-cell;
width: 45%;
background: #41444A;
vertical-align: top;
color: #FFFFFF;
}