//vertically centers the entire main content
function vertically_center()
{
  var window_height = window.innerHeight || document.documentElement.clientHeight || document.body.clientHeight;
  var my_div = document.getElementById('container');
  
  //we're goign to adjust the marginTop, so we get half the difference in viewing and element height
  
  if(my_div)
  {
    var new_offset = (window_height - my_div.offsetHeight)/2;
    
    //make sure we don't get a negative margin
    if(new_offset < 0)
    {
      new_offset = 0;
    }
    new_offset = new_offset + 'px';
    
    
    my_div.style.marginTop = new_offset;
  }
}