I don't think we can add this as an optional setting right now, but here is some Custom CSS you can use to create grid lines: td>img[src*="battleship"], td>img[src*="_water"],td>a>img[src*="battleship"], td>a>img[src*="_water"] {border:1px solid ;} (Simply copy and paste that into the text box on the Custom CSS page linked above, and it will add the grid lines when you play the game.)

You can change the "124358" part to another hex color code to change the border color (FFF for white, 111 for black, for example). I hope this helps :)