Hi. Thanks !
You discovered a bug :)
gridSize is changable (for now) from editing the gridSize variable. The problem is that CSS was set fixed to grid=20 .
The fix (if you are in a hurry) is to add two lines in the initGrid() function :
function initGrid() {
// Set the grid template columns dynamically based on gridSize
document.getElementById('grid').style.gridTemplateColumns = `repeat(${gridSize}, 20px)`;
....rest of the initGrid(){ function..