dark transparent overlay with flat bootstrap theme

A dark transparent screen covers my whole wp page. There is an x and a darker strip in the center. If I click the x, it disappears, but if I refresh or go to a new page there it is again. I’ve deactivated all plug-ins and switched themes to try to isolate the problem. This dark overlay is only present when my flat bootstrap theme is active. What is this? I have used this theme many times before and have never encountered this.