title: Make div appear and change the whole html to be darker
body: <p>I have a div and after I click a button, I would like the div to appear (which I can do), but I would like the whole background to become darker too (this is inline with overlays).</p>
  
  <p>I've tried using opacity - I change the opacity of the whole html with jQuery, i.e. <code>$('html').css('opacity','-0.5');</code> and change back the opacity of the div to normal, but for some reason, the opacity of the div stays the same (0.5). 
  I don't quite like the opacity since actually it doesn't make the background darker (rather lighter). </p>