But if you're after some kind of masking the whole page, this is usually done by adding a separate div with this set of styles:
position: fixed;
width: 100%;
height: 100%;
z-index: 1000; /* some high enough value so it will render on top */
opacity: .5;
filter: alpha(opacity=50);
...where the first three numbers are the red, green and blue values for your background colour, and the fourth is the 'alpha' channel value, which works the same way as the opacity value.