Theme Background Question

I just installed the free flat theme on my website. I am able to change the color of my side menu. But, I am not able to change the background color of the site. Under the settings where the side menu is there is a color picker for the background color as well but it doesn’t change color. The background just stays the default white color even though I pick a different color using the color picker by double clicking on the color that I want to use just like the side menu. How else can I change the background color of my site using this theme?

The page I need help with: [log in to see the link]