set background black with 50% opacity

Question

So, I have this div #welcome that runs this code

if ($.cookie('20120129') != '1') {
    $('#welcome').slideDown('slow');
    $.cookie('20120129', '1', { expires: 20 }); 
}

#welcome{
  position: absolute; z-index:100;
  background: #fff; color: #000;
  border: 1px solid black;
  display: none;
  width: 1000px;
  margin: 0 auto;
}
#welcome p{padding: 100px;}

I was wondering how to set a background layer between #welcome and the page with an 50% opicity, like thickbox/colorbox...

Answer 1

Add a fixed overlay that is hidden by default and shown when you need it. You can either add this to your HTML structure yourself, or use Jquery to add it. Personally I would add it to the HTML structure.

The .overlay element must have a z-index lower than #welcome but higher than any other elements it must cover:

.overlay {
    background-color: #000;
    bottom: 0;
    display: none;
    left: 0;
    opacity: 0.5;
    filter: alpha(opacity = 50); /* IE7 & 8 */
    position: fixed;
    right: 0;
    top: 0;
    z-index: 99;
}

Updated Jquery to add / show overlay div:

//add overlay if it does not exist
if( $('.overlay').length == 0 ){
    $('body').append('<div class="overlay"></div>');
} 
if ($.cookie('20120129') != '1') {
    $('.overlay').show();
    $('#welcome').slideDown('slow');
    $.cookie('20120129', '1', { expires: 20 }); 
}