I just found the following useful, answered question:
The above answer suggests how to change the colour of the lines but that solution is not sufficient for me. What I also need to know is this: is it possible to change the thickness of the grid lines?
I find the grid lines too thin, even when set to black. Strangely, the lines are easier to see when zoomed out but when I zoom in, the lines get progressively fainter until I can not see them anymore.