Hi D,
In SmartBuilder v4 you can have multiple Masters. So in this case, you'll want one Master that shows up on top (this might be one of our Master Templates) so that any popup resources show above the page level content, and then a second Master that has your background, which is set to be below your page level content. Here's a quick video showing the process:
https://share.smartbuilder.com/public/support/master-background-example.mp4Hope that helps!