body {
    margin: 0 auto;
    width: 90%;
}

em {
    color: red;
    font-weight: bold;
}

.photo-list {
    max-width: 800px;
    list-style-type: none;
    display: flex;
    justify-content: space-around;
    flex-wrap: wrap;

}

.photo-list li {
    margin: 1em;
    padding: 0.5em;
    border-radius: 8px;
    transition: background-color 0.5s;
}

.photo-list li:hover {
    background-color: silver;
}

.photo-list li.selected {
    background-color: forestgreen;
}