body {
    box-sizing: border-box;
}
#sudoku-container {
    margin: auto;
    width: 500px;
    height: 534px;
    overflow: hidden;
    background-color: lightblue;
    text-align: center;
}
button {
    width: 100px;
    padding: 5px 20px;
    margin-top: 7px;
    font-size: 12px;
    background-color: #ffefd5;
    border: 1px solid black; 
    cursor: pointer; 
}
#board {
    margin: 7px auto 0px auto;
    border: 3px solid black;
    border-collapse: collapse;
}
#board tr:nth-child(3n) {
    border-bottom: 3px solid black;
}
#board td {
    width: 50px;
    height: 50px;
    border: 1px solid black;
    cursor: pointer;
}
#board tr:nth-child(odd) td:nth-child(odd),
#board tr:nth-child(even) td:nth-child(even) {
    background-color: #ffefd5;
}
#board td:nth-child(3n) {
    border-right: 3px solid black;
}
@media screen and (max-width: 510px) {
    #sudoku-container {
        width: 250px;
    }
}