Could it be the theme that I'm using (adaptable)?
Probably, yes. You can verify for yourself, by switching to a standard theme, such as "Clean" or "More", and then viewing your JMatch again.
In order to fix this issue then, you have to establish which CSS from the Adaptable theme is affecting the white boxes and images in your JMatch, and then override that with your own CSS inserted into the Hot Potatoes exercise.
Most modern browsers have a way to display and adjust the CSS on a webpage. For example, Chrome has a "Developer tools" window, and Firefox has a "Web console" window.
Once you have determined which CSS you need to override the Adaptable theme, you can either insert it into the "Custom CSS" box of the Adaptable theme, or you can add it to each individual Hot Potatoes exercise that you create.