#sudoku-grid {
    border-collapse: collapse;
}

#sudoku-grid > div {
    border: 1px solid rgb(209, 213, 219);
}

#sudoku-grid > div:nth-child(n):nth-child(-n + 9) {
    border-top: 2px solid rgb(75, 85, 99);
}

#sudoku-grid > div:nth-child(9n + 1) {
    border-left: 2px solid rgb(75, 85, 99);
}

#sudoku-grid > div:nth-child(9n) {
    border-right: 2px solid rgb(75, 85, 99);
}

#sudoku-grid > div:nth-child(n + 73) {
    border-bottom: 2px solid rgb(75, 85, 99);
}

#sudoku-grid > div:nth-child(9n + 3),
#sudoku-grid > div:nth-child(9n + 6) {
    border-right: 2px solid rgb(75, 85, 99);
}

#sudoku-grid > div:nth-child(n + 19):nth-child(-n + 27),
#sudoku-grid > div:nth-child(n + 46):nth-child(-n + 54) {
    border-bottom: 2px solid rgb(75, 85, 99);
}