:root {
    --cell-size: 120px;
    --cell-gutter: 20px;
}

/*

(var(--cell-size) + var(--cell-gutter)) * 3

*/

#grid {
    display: flex;
    flex-direction: row;
    flex-wrap: wrap;

    align-content: space-between;
    justify-content: space-between;

    width: calc((var(--cell-size) + var(--cell-gutter)) * 3);
    height: calc((var(--cell-size) + var(--cell-gutter)) * 3);
}

.cell {
    height: var(--cell-size);
    width: var(--cell-size);
    border: 4px solid white;

    cursor: pointer;

    font-size: 60px;
    display: flex;
    align-content: center;
    justify-content: center;
}