Sidebar same length as frame

My problem is the following:

Have a look at my site https://fan-sites.org/audrey-tautou – if you scroll down, you will see that the sidebar and the main frame do not have the same length. They never do as they always change of course, if the content in the sidebar or the frame gets more / less.

How can I manage the sidebar to be as long as the frame (if the frame is longer than the sidebar)? Is there something I can do about it? Or not?

Thanks for support!