Gridlines: not all or nothing?

[Ludovico]

A friend asked me to look at a worksheet which has gridlines showing in most, but not all cells. He wants to restore them to all cells. Checking "Tools, Options, View, Gridlines" off and on again doesn't do it. How can I help him get them back?

 

[AP]

Did you try to fiddle with the background colors? The gridlines may be set to be the same as that

 

[Ludovico]

Of course.
Thanks, AP.