body {
    background: #1a1a1a;
    color: #fff;
    font-family: sans-serif;
    display: flex;
    justify-content: center;
    align-items: center;
    height: 100vh;
    margin: 0;
}
.container { text-align: center; }
.sudoku-grid {
    display: grid;
    grid-template-columns: repeat(9, 40px);
    grid-template-rows: repeat(9, 40px);
    gap: 1px;
    background: #444;
    padding: 2px;
    border: 3px solid #fff;
    margin-bottom: 20px;
}
.sudoku-grid input {
    width: 40px;
    height: 40px;
    text-align: center;
    font-size: 18px;
    background: #2a2a2a;
    color: #fff;
    border: none;
}
.sudoku-grid input:nth-child(3n) { border-right: 2px solid #fff; }
.sudoku-grid input:nth-child(9n) { border-right: none; }
/* Row grouping borders */
.sudoku-grid input:nth-child(n+19):nth-child(-n+27),
.sudoku-grid input:nth-child(n+46):nth-child(-n+54) {
    border-bottom: 2px solid #fff;
}
.controls button {
    padding: 10px 20px;
    margin: 0 10px;
    font-size: 16px;
    cursor: pointer;
    background: #4ab3f4;
    border: none;
    color: white;
    border-radius: 4px;
}