body {
    padding: 20px;
    margin: 0;
}

#field {
    position: relative;
    padding: 0;
    width: 600px;
    height: 600px;
    display: grid;
    border: 1px solid #000;
}

#field span {
    display: flex;
    width: 100%;
    height: 100%;
    text-align: center;
    box-sizing: border-box;
    border: 1px solid #000;
    cursor: pointer;
    justify-items: center;
    justify-content: center;
    align-items: center;
    font-size: 20px;
    background-image: url(images/grass.png);
    background-size: 100% auto;
    color: #fff;
}