.square{
  display:flex;justify-content:center;align-items:center;
  width:3rem;height:3rem;border-radius:.25rem;
  font-size:1.25rem;font-weight:bold;user-select:none;
  cursor:pointer;transition:background-color .15s,color .15s}
.square.gray{background:#334155;color:#e2e8f0}
.square.yellow{background:#facc15;color:#1e293b}
.square.green{background:#10b981;color:#fff}