In Excel 2016 for Mac, I clicked on the Excel menu and then clicked "Settings" ...
Then I clicked the View icon.
after which I checked the Gridlines checkbox next to the Color Picker.
Regardless of whether the Gridlines os checkbox is checked or not, if you change the color in the Color Picker drop-down menu, the borders of your cell will become the same. (I believe that the change occurred after I quit Excel and opened the document the next day to continue working on it, but I can’t remember exactly.)
Changing the color selection in automatic mode will return the borders of your cell to the default (black) on the fly.
NB Since I am a beginner, I can’t insert the screenshots that I prepared ahead of time. 😠
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