Wordpress.Com CSS customization Help?

I am editing the CSS fr my wordpress.COM blog. I changed the color of the page wrapper so there is a blue border around everything, but now, where the last sidebar widget ends, it turns all dark blue (the color of the page wrapper.) I added a bunch of breaks in the widget to get the lighter sidebar color to extend down, but now, when I post something new, it gets messed up and my footer image goes down, and you can see the dark blue where the last post ends. What can I do so that when I post something new and the entire blog height changes, how to get the footer picture along with the continuous light blue sidebar color to move up and align with the last post. so, basically, I want the sidebar border and the footer image to stay together and line up with everything else as posts change. I realize this is kind of confusing.......my blog is ringling9.wordpress.com (I dont have the css upgrade yet, I want to figure out my design first.)