Using Numbers, I no longer see any differentiation indicating which sheet I am actively using in a file with multiple sheets. All sheet tabs are visually the same. Any solution? Using Catalina 10.15.7 on my Mac (can no longer update due to age).

Assuming you have already tried restarting your Mac, you could try signing in as another user and seeing if the problem persists there too. Also, deleting Numbers and reinstalling it from the Mac App Store sometimes clears up problems.

SG

Is this what you are describing: https://discussions.apple.com/thread/252642782

If it is, unfortunately the person with the problem never wrote back if they solved it or how.