#mainImg {
    display: flex;
    justify-content: center;
    align-items: center;

    margin: 20px;
}

#mainImg > img {
    width: 250px;
    height: 250px;
}

#thumImg {
    margin-top: 20px;
}

#thumImg > img {
    width: 100px;
    height: 100px;

    margin: 4px;
}

#thumImg > img:hover {
    cursor: pointer;

    margin: 0px;
    border: 4px solid blue;
}