Paul, I tested whether the color contrast is the problem.
I created 2 Excel files each containing a custom tab with 2 large buttons. In one of the 2 files I placed a separator between the 2 buttons. See attached images and files.
Then I opened both file on the computers where I can see and where I cannot see separators.
On the computer where I can see separators, when toggling between the 2 files, I can see that the separator is changing the distance between the 2 buttons.
On the computer where I cannot see separators the distance between the 2 buttons remains the same.
This result indicates to me that the color contrast is not the problem.
