style.css

.sudoku {
    width: 800px;
    height: 800px;
    background: #fff;
    border: 3px solid black;
    display: grid;
    grid-template-columns: repeat(3, 1fr);
    grid-template-rows: repeat(3, 1fr);
    margin-right: 30px;
    touch-action: manipulation;
}

.sudoku-block {
    display: grid;
    grid-template-columns: repeat(3, 1fr);
    grid-template-rows: repeat(3, 1fr);
    border: 2px solid black;
}

.sudoku-cell {
    display:flex;
    justify-content:center;
    align-items:center;
    border: 2px solid grey;
    line-height: 100%;
}
.sudoku-cell > span {
    font-size: 36px;
    user-select: none;
}

.active-sudoku-cell {
    background: #ffe97b;
}

.sudoku-cell-error {
    border-color: red;
}
.sudoku-clue {
    background: #ccc;
}

.sudoku-pencilmarks {}
.sudoku-pencilmarks > ul {
    padding: 0;
    margin: 0;
    display: grid;
    width: 100%;
    height: 100%;
    grid-template-columns: repeat(3, 1fr);
    grid-template-rows: repeat(3, 1fr);
}
.sudoku-pencilmarks > ul > li {
    display: flex;
    justify-content:center;
    align-items:center;
    font-size: 16px;
    user-select: none;
}

body {
    display: grid;
    grid-template-columns: 810px 1fr;
    background: #c3dff6;
    font-family: Verdana, sans-serif;
}