index.css

body {
        padding: 0;
        margin: 0;
        background-color: burlywood;
}

#game-board {
        display: inline-grid;
        grid-template-columns: repeat(15, auto);
}

.outer-content {
        display: table;
        position: absolute;
        top: 0;
        left: 0;
        height: 100%;
        width: 100%;
}

.middle-content {
        display: table-cell;
        vertical-align: middle;
}

.inner-content {
        width: 540px;
        margin-left: auto;
        margin-right: auto;
        display: block;
}

.cross {
        background-color: transparent;
        width: 36px;
        height: 36px;
        padding: 0;
        margin: 0;
        position: relative;
        border-radius: 50%;
}

.cross:hover {
        background-color: #f5deb3cc;
}

.vl {
        height: 100%;
        border-left: 1px solid black;
        margin-left: 18px;
        position: absolute;
}

.hl {
        width: 100%;
        border-top: 1px solid black;
        margin-top: 18px;
        position: absolute;
}

.dot {
        height: 10px;
        width: 10px;
        background-color: black;
        border-radius: 50%;
        margin-left: 13px;
        margin-top: 13px;
        position: absolute;
}

.chess {
        height: 30px;
        width: 30px;
        border-radius: 50%;
        margin-left: 3px;
        margin-top: 3px;
        position: absolute;
        display: flex;
        justify-content: center;
        text-align: center;
        flex-direction: column;
}

.chess-black {
        background-color: black;
        color: white;
        box-shadow: inset 3px 3px 5px #aaa7;
}

.chess-black[last-move] {
        box-shadow: inset 3px 3px 5px #aaa7, 0px 0px 0px 6px #f5deb3cc;
}

.chess-white {
        background-color: white;
        color: black;
        box-shadow: inset -2px -2px 2px #7777;
}

.chess-white[last-move] {
        box-shadow: inset -2px -2px 2px #7777, 0px 0px 0px 6px #f5deb3cc;
}

@keyframes combo-anime {
        0%      { background-color: transparent; }
        50%     { background-color: #ff6347aa; }
        100%    { background-color: transparent; }
}

.combo {
        animation-name: combo-anime;
        animation-duration: 2s;
        animation-iteration-count: infinite;
}