My background does not extend till footer in post pages

Hi!
I am facing this problem. Please guide me.
When the post page is opened the sidebar background doesn’t extend all the way down till footer. When I give a height to #bottombody css style then it extends to the specified height. That height will become irrelevant if there are many comments. I want to extend the #bottombody background till footer depending on the comments’ number.
Bear with me if I sound silly.

Please help. Thanks in advance.