Set height to 100%

The easiest way to get full height is

#content {
      position:absolute;
      top:0;
      bottom:0;
  }

You can also use the position:fixed, but that will not scroll with the window.

You are missing this:
html { height: 100%; }

It won’t help you though because setting 100% height to element sets it to full height of parent element, not parent element’s content. Absolute positioning won’t help you ether because it will mess up layout if post’s content requires more height than is available to it.