Oh, I see. Yes, those are there. When I read the feature description, I thought you could change the light blue lines to be some other colour.
The actual issue I'm having isn't so much with the lines, but with the 'x' marks. On my computer or my phone (i.e. on modern display hardware) they're great, but when I try to play this on my Nintendo 2DS XL, they're nearly impossible to see against the white/cream background. I have to expect it'd be worse on a real GBC. So I was hoping that if you could change the gridlines maybe it would also change the 'x' marks, which was, of course, a long shot. :)
I'm very nearly done, though -- just mopping up the last few extra puzzles -- and it's a fantastic game even despite that. I can't wait for the sequel!