By default, the sidebar in WordPress only goes down as far as the content in the sidebar goes down. Every now and then, I have a design that calls for the sidebar background color to extend down the full length of the page. I’ve tried making different divs 100% height, but that never seems to work for me. What I have found to work is to make a background image for the whole content area and then use “repeat-y” to extend it down. This automatically makes the sidebar the full length of the content area. My background image would look something like this:
You’ll see it’s just a couple pixels high, but the full width of the content area. Then I’ll add my CSS, making sure to select only the pages that have a sidebar.
.content-sidebar #content-sidebar-wrap {background: url(images/content-bg.png) repeat-y;}
Hope it works for you as well!