Need visual cue to distinguish between each instance of a program

Windows 10 - I have two instances of an application program open at the same time, each in their own window. The application program in this case is Family Tree Maker for Windows, Version 2019. I want to do all my updating to instance A and use instance B as a reference. I want an "obvious" visual cue to help me remember which window is the one I make changes in (green theme) and which window is reference only (red theme). My first thought was to look at the accessibility settings. I think I need to be able to customize settings (such as color) at a program level and not at the Windows OS level.

Can you offer alternative solutions for accomplishing the situation I've defined?