It was just too hard to see things on my phone.
I hadn't used this larger font size earlier because it caused the
squares to stretch tall (no longer as squares) and squashing flat the
mini grids without characters in them.
But here I found I can avoid that problem by setting line height to 0.
display: flex;
justify-content: center;
align-items: center;
display: flex;
justify-content: center;
align-items: center;
+ font-size: 8vmin;
+ line-height: 0;
font-weight: bold;
border-bottom: 1px solid #999;
border-right: 1px solid #999;
font-weight: bold;
border-bottom: 1px solid #999;
border-right: 1px solid #999;