.optimal-tile {
    position: absolute;
    width: 50px;
    height: 50px;
    box-sizing: border-box;
    pointer-events: none; /* So that tiles can be placed over them */
    z-index: 1;
    border: 2px solid transparent;
    transition: background-color 200ms linear;
}

#grid-content.show-optimal .optimal-tile {
    background-color: rgba(213, 180, 0, 10%);
}