What you're looking for is just:
background-color: rgba(0, 0, 0, .8);
z-index: 1000;
Z-index can be any value, which is greater than background's z-index. If you haven't used it anywhere else, it's the same effect if its 1 or 9999.
The last '.8' is transparency
, which is a value from 0.0 to 1.0, where 0 is invisible and 1 is fully visible.
But when looking at this, I think the effect you're looking for is contained inside <figure id="thumbnail" ...>
, which is an image which fills the whole screen width.
At this point I can't give you any better advice because I don't know what exactly you're going for. Try providing code of what you've already done and what's not working like you want it to.