It's very distracting and complicates color assessment withhaving the border around the active window being white when the non-active window is dark gray. Perhaps some other means could be used to identify the active or non-active window.
You can customize the color of the background by going to Photo Mechanic's Preferences and going to the Preview section. The relevant colors to pick would be the 'Background' color and the 'Selected background color'.
If not by the background color, do you have any suggestions for the best way for Photo Mechanic to indicate what the selected image is?