another z-index value, than my “auto”.
Within Firebug you can test it with positive or negative values, until you see the overlay. In my case, with my theme it only works if I quasi delete the original z-index value of “-1997” and change it therefor to “auto”.
I figured this all out with Firebug, give it a chance, too 🙂
I faced this same issue. Turns out this was because a background-color was added to the body tag via CSS. Removing the background-color property from the body tag fixed this issue! Now not only the background overlay displays correctly, but its opacity setting is also applied!
Edit: If you still can’t see the overlay, try reducing the background image opacity to less than 100%