Open theme tweaker, type in "grid".
Apart from the colour, the only other thing that can be changed is the blend /alpha, by clicking the "normal" line.
I've found that blend mode normal, alpha 1 is best for visibility, and since I use a lot of colours on my arrangements, the best contrast I seem to find is a light grey (RGB = 150 150 150).
It still gets a bit hard to see over lighter colours, but visible if you zoom in.
