body {
    font-family: sans-serif;
    text-align: center;
    margin: 20px;
}

table {
    border-collapse: collapse;
    margin: 0 auto;
}

td {
    width: 40px;
    height: 40px;
    text-align: center;
    border: 1px solid #999;
    font-size: 20px;
    font-weight: bold;
}

td.input {
    font-weight: normal;
}

input {
    width: 38px;
    height: 38px;
    text-align: center;
    font-size: 20px;
    border: none;
}

td:nth-child(3n) {
    border-right: 2px solid black;
}

td:nth-child(3n-2) {
    border-left: 2px solid black;
}

tr:nth-child(3n) {
    border-bottom: 2px solid black;
}

tr:nth-child(3n-2) {
    border-top: 2px solid black;
}

#controls {
    margin-top: 15px;
}