* {
    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;
    background-color: white;
}

.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 auto;
    background-color: gray;
}

#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;
}

.white {
    color: white;
}