#47 Grids larger than 3x3

[Kazimierz Król]

The numbers 10 and above touch the cell borders, so it looks a bit messy.

I think it would be better to use letters instead of numbers above 9, like 8, 9, A, B, C, D, E, F, G, or use letters A - P for everything (only for grids larger than 3x3, and possibly in a configurable way).