the background of what you could consider to be the page or post area is in div #main:
either add a solid color to the style of #main around line 440 in style.css of 2010 theme:
#main {
overflow: hidden;
padding: 40px 0 0 0;
background-color: #123456;
}
this color would be a solid background including the sidebar area.
or make a background image (horizontal slice, height 10px or less) width 940px with separate color regions for the main and the sidebar area (and/or a divider line) saved in the /images folder and reference it in style.css (for instance):
#main {
overflow: hidden;
padding: 40px 0 0 0;
background: #123456 url(images/main_bg.png) center top repeat-y;
}
if you should use the page template without sidebar, you need to change the above style in style.css for this.