client/themes/material/css/grid.css

/* Grid */
ul.grid > li {
    background-size: cover;
    width: calc(25% - 0.2em);
    padding: 0;
    padding-bottom: 25%;
    float: left;
    position: relative;
    background-repeat: no-repeat;
    background-position: center center;
    box-sizing: border-box;
    margin: 0.1em;
}

ul.grid.active > li:hover {
    cursor: pointer;
}

ul.grid > li > nav {
    position: absolute;
    bottom: 0;
    color: white;
    background-color: rgba(0, 0, 0, 0.5);
    width: 100%;
    padding: 0 1rem;
    box-sizing: border-box;
    white-space: nowrap;
    text-overflow: ellipsis;
    overflow: hidden;
    font-size: 2rem;
    line-height: 5rem;
}

@media screen and (max-width: 1024px) {
    ul.grid > li {
        width: calc(33.33% - 0.2em);
        padding-bottom: 33.33%;
    }
}

@media screen and (max-width: 640px) {
    ul.grid > li {
        width: calc(50% - 0.2em);
        padding-bottom: 50%;
    }
}

ul.grid:after {
    content: "";
    display: block;
    clear: both;
}

ul.grid.padded {
    padding: 0;
}