overlay.js

$(function() {

  // create a <div> to hold the overlay styles ...
  $("body").append("<div id='overlay'></div>");
  
  // get the height of the document ...
  var docHeight = $(document).height();
  
  // add some styling to the overlay.
  $("#overlay")
    .height(docHeight)
    .css({
      'top': 0,
      'left': 0,
      'width': '100%',
      'position': 'fixed',
      'background-color': 'black',
      'opacity' : 0.1,
      'z-index': 5000
    });
    
});