I agree. Even with 0.02-inch thickness and 170-170-170 (light gray)
color, the grid lines come on too strong. I'll get around to trying
0.01 inch one of these days, and if that's still too much or LibreOffice
doesn't allow it, a yet lighter gray.
This past Thursday (02/03/2022), I I finally got around to trying to adjust the grid lines
down to a thickness of 0.01 inch, and the color to light gray (decimal RGB =
(191,191,191)). The resulting printed graph is excellent.