Sidebar Problem

Hi, so far I’ve had no problems developing my blog’s look on the Simpla theme by Phu, however I’ve hit a little snag, as you might be able to see on my blogs front page…

https://blabj.com

(For those of you who can’t quite see what I’m getting at, check the sidebar… It has a rather nice blue gap underneath it.)

So far my attempts to rectify the problem have been largely (read: completely) unsuccessful, and I’m getting a little bit irritated from the look of it, and as a result it’s making me less enthusiastic about editing anything else because of the look of the page..

I would like to know whether I have a CSS problem, or if it’s something that’s just not properly rendering, and if so some advice on how to go about fixing the problem.

I’ll also include some links back to the HTML and CSS validator for you, with the source shown, just to show you what I’m up against, HTML (note: ignore my over-zealous commenting, I was having a few problems with loose and open <divs>) CSS.

Thanks for any replies I get in advance!