.board {
    display: grid;
    grid-template-columns: repeat(3, 1fr);
    grid-gap: 5px;
    width: 300px;
    margin: auto;
    border: 2px solid black;
    padding: 5px;
}

.square {
    height: 90px;
    background-color: lightgray;
    display: flex;
    justify-content: center;
    align-items: center;
    font-size: 2rem;
    cursor: pointer;
    border: 1px solid black;
}

.row .square:not(:last-child) {
    border-right: 1px solid black;
}

.board .row:not(:last-child) {
    border-bottom: 1px solid black;
}