hmmm... I guess it's have looks like this:

#ground {
background-color:#111;
opacity: 0.65;
filter:alpha(opacity=65);
position:absolute;
z-index: 9001;
top:0px;
left:0px;
width:100%;
}
#popUpDiv {
position:absolute;
background-color:#eeeeee;
width:300px;
height:300px;
z-index: 9002

You can try learn more on html css tutorial. Seemed I met this info there :)