body {
    display: flex;
    height: 100vh;
    margin: 0;
}

table.sudoku{
    border: medium solid black;
    border-collapse: collapse;
    margin: auto;

    td{
        border: thin solid lightgray;
        height: 3rem;
        width: 3rem;
        vertical-align: middle;
        text-align: center;
    }

    td:nth-child(3n){
        border-right: medium solid black;
    }
    tr:nth-child(3n){
        border-bottom: medium solid black;
    }
}