Hi mate,
the CSS isn’t the problem here, it’s an HTML thing. You’ll notice in the second image the blue bit that reaches the bottom actually surrounds the sidebar as well. So both the content and the sidebar are surrounded by a <div> that has a blue background. It’s the sidebar that pushes that blue box down, not the content. In your example you are targetting only the content so the CSS is doing the right thing by making the content box white (#ffffff). You should see if there is a <div> that surrounds both your content and sidebar; if there is make the background of that white.