body { background:#222; color:#fff; text-align:center; }
#board { display:grid; grid-template-columns:repeat(3,100px); gap:5px; justify-content:center; margin-top:20px; }
.cell { width:100px; height:100px; background:#555; display:flex; align-items:center; justify-content:center; font-size:2em; cursor:pointer; }