Prototype fullscreen background

When I look at my desktop design prototype after sharing it in prototype mode I get a black box around my pages in full screen. What's that all about? Looks very odd and not as intended. If my page background is white I want the WHOLE background in the preview to be white, just like it would if I made the page in Dreamweaver and previewed it. Now XD is making each page look like a box in the middle of the screen with black all around it. I keep getting questions from clients if I can show it with the correct bg color understandably, but there's no option for that as far as I can see? I keep getting the black background all the time.