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I've seen a similar issue where I used a light theme (default? Active4d), and all of a sudden while in the app, I get switched to a dark theme.  Perhaps this is related?

Seems like a key combination, but I can't track it down. Happens a lot.  It happened just now by simply moving the window by clicking the title bar and moving the mouse.  I restarted the app and could not reproduce it. (OS 10.3.1)