How can I remove the shadow on the spreadsheet headers and row indicators?

English
#1

I would like the columns A, B, C, … to show a flat look, not to have a pseudo 3d shadow. Same for row indicators 1,2,3,…
Thank you

#2

A similar question was asked 3 weeks ago here:

Disable the “3D” effect on cells Name header in calc

#3

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