Make div appear and change the whole html to be darker
<p>I have a div with embedded youtube. After I click some button, I would like the div to appear (I know how to achieve this), but I want the whole background to become darker. (This is inline with overlays.)
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>

<p>Any ideas how to make the background darker?</p>
