ul {
    margin: 0;
    padding: 0;
}

li {
    list-style-type: none;
    display: inline-block;
    width: 10em;
    height: 10em;
    padding: 0;
    margin: 1em 0.3em;
    overflow: hidden;
    border: solid 1px black;
    border-radius: 0.2em;
}
li:hover {
    border-color: #888;
    background-color: #ddf;
}

li h1 {
    text-align: center;
    font-size: 1em;
    line-height: 1em;
}

img {
    display: block;
    max-width: 100%;
    max-height: 6.8em;
    width: auto;
    height: 6.8em;
    margin: 0 auto;
    
    /*filter: invert(1) opacity(0.8);*/
    filter: brightness(0) opacity(0.8);
}

body >h1,
form {
    text-align: center;
}