* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
  }
  
  body { font-family: monospace }
  h1 { text-align: center }
  
  .cell {
    height: 100px;
    width: 100px;
    display: flex;
    align-items: center;
    justify-content: center;
    font-size: 80px;
    border-right: 2px solid black;
    border-bottom: 2px solid black;
    cursor: pointer;
  }
  
  .cell:nth-child(3n) { border-right: none }
  .cell:nth-child(6) ~ .cell { border-bottom: none }
  
  .board {
    width: 300px;
    display: flex;
    flex-wrap: wrap;
    margin: 4rem;
  }
  
  .white { color: white }
  
  #gameEnd {
    position: fixed;
    display: none;
    width: 100%;
    height: 100%;
    top: 0; right: 0; bottom: 0; left: 0;
    background-color: black;
  }
  
  #gameEnd p, #gameEnd input {
    position: fixed;
    top: 45%; left: 45%;
    transform: translate(-45%, -45%);
    text-align: center;
  }
  
  #gameEnd p { margin-top: 2rem }