#main {
    width: 408px;
    height: 408px;
    background: white;
    border: 2px solid gray;
    margin: 0 auto;
    overflow: hidden;
}

#con {
    position: relative;
    top: -408px;
    border-collapse: collapse;
    width: 100%;
    height: 400px;
}

h2 {
    text-align: center;
}

.row {
    height: 100px;
    width: 100%;
}

.white_cell, .black_cell {
    height: 100px;
    width: 100px;
    float: left;
    border: 1px solid rgb(54, 74, 129);
}

.black_cell {
    background: black;
}

.btn {
    width: 100%;
    text-align: center;
}

#start {
    margin: 20px auto;
    width: 150px;
    height: 50px;
    border-radius: 10px;
    background: yellowgreen;
    color: white;
    line-height: 50px;
}