Here is a possible work around to partially do this. You can upload a custom board background image that has 19x19 squares aligned with the stones such that the stones are centered on the squares. If you play many games across varying board sizes, it will be kind of annoying to switch the image for each game, but it’s still doable.
However, the black grid lines are still overlaid and could be distracting. Maybe your background image could use a different color for the square boundaries to contrast. Or perhaps, there should be a customization option to change the color of the grid lines (or remove them altogether). EDIT: @AdamR showed that this can already be done below.
EDIT2: See below where I do this