body {
  margin: 0;
  display: grid;
  place-items: center;
  background: #58b2de;
  font-family: system-ui;
}

h1 {
  color: #fff;
  font-size: 44px;
  margin: 10px 0 16px;
  font-weight: 900;
  text-align: center;
}

.board {
  width: min(320px, 70vw);
  height: min(320px, 70vw);
  display: grid;
  grid-template: repeat(3, 1fr) / repeat(3, 1fr);
  border: 10px solid rgba(255, 255, 255, 0.25);
  border-radius: 12px;
  overflow: hidden;
  background: rgba(255, 255, 255, 0.08);
}

.cell {
  appearance: none;
  -webkit-appearance: none;
  margin: 0;
  padding: 0;
  outline: 0;
  border: 12px solid rgba(255, 255, 255, 0.25);
  background: rgba(255, 255, 255, 0.14);
  display: flex;
  align-items: center;
  justify-content: center;
  font-size: clamp(90px, 16vw, 90px);
  font-weight: 900;
  line-height: 0;
  border-radius: 5px;
  color: #7cd0ff;
  cursor: pointer;
}

.cell.filled {
  background: rgba(255, 255, 255, 0.95);
  cursor: default;
}

button {
  border: 3px solid rgba(255, 255, 255, 0.75);
  background: rgba(255, 255, 255, 0.2);
  color: #fff;
  padding: 12px 18px;
  border-radius: 10px;
  font-weight: 900;
  font-size: 18px;
  cursor: pointer;
  margin-top: 16px;
}

.overlay {
  position: fixed;
  inset: 0;
  display: none;
  place-items: center;
  background: rgba(0, 0, 0, 0.35);
}

.overlay.show {
  display: grid;
}

.modal {
  background: #fff;
  border-radius: 14px;
  padding: 18px 16px;
  width: min(320px, 90vw);
  text-align: center;
}

.modal h2 {
  margin: 0 0 10px;
  font-size: 40px;
  font-weight: 900;
  color: #1d4f66;
}