Overlay on entire page behind popup

Hi there!

I love this plug-in, but am just struggling with getting it 100% right. I wanted a popup to appear once a user clicks a button (to sign up to access a free download). I’ve got the popup working, but the overlay takes over the entire page behind it (image or colour). I was keen for it to have a transparent background, or transparent with a small % of black to darken it. I’m sure there’s a simple solution but I can’t figure it out! Are you able to tell me what I need to do (I’ve looked on the BB Facebook Group and online for answers but couldn’t find one!)

Thanks very much
Zoe