You didn't implement the sticky footer correctly but that on its own will not be enough to make the middle section stretch as the sticky footer can only have one wrapper and that must hold all your content except the footer. No other wrappers allowed because of the way that css works.
You can provide the full length middle section using my overlay technique (ie7+) which will overlay the background colour on that middle section. However you will have to follow the instructions exactly which you have not done for the footer already
It would be better for you to build your layout around the sticky footer structure rather than the other way around as there is no other way to do this.
Here is an example showing the effect I think you are looking for. Be aware that you could have the layout as a max-width layout instead of fixed width but that the content in the header and footer would still need to remain at a fixed height otherwise it will overlap.
As you have an awful lot of code in that site already you will have to be careful that the structure matches my example exactly.
Please read the faq on the sticky footer again and on 100% height as they hold important concepts that need to be understood before you can manipulate or make changes to the layout.