style.css

* {
    box-sizing: border-box;
}

body {
    /* background-color: #616161; */
    /* background-color: #362c3f; */
    background-color: #4f4547;
    position: relative;
    margin: 0;
    width: 100vw;
    height: 100vh;
    display: flex;
    justify-content: center;
    align-items: center;
}

.board {
    min-width: 700px;
    min-height: 700px;
    border-radius: 10px;
    padding: 16px;
    background-color: #1e1e1e;
    color: #e1e1e1;
    display: flex;
    flex-wrap: wrap;
}

.box {
    display: inline-block;
    position: relative;
    width: calc(100%/3 - 32px);
    margin: 16px;
    border-radius: 10px;
    font-size: 8em;
    line-height: 0;
    background-color: #fff1;
    cursor: pointer;
}

.box i {
    visibility: hidden;
    position: absolute;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);
}

.box i::before {
    text-shadow: 0 10px 10px rgba(0, 0, 0, 0.2);
    transition: transform 200ms;
}

.box:hover i::before {
    text-shadow: 0 16px 13px rgba(0, 0, 0, 0.2);
    transform: scale(1.1);
}

.box i[show] {
    visibility: visible;
    animation: sign-show 400ms ease forwards;
}

@keyframes sign-show {
    0% {
        font-size: 0;
    } 50% {
        font-size: 10rem;
    } 100% {
        font-size: 8rem;
    }
}

.gameover-overlay {
    display: none;
    position: absolute;
    top: 0;
    bottom: 0;
    right: 0;
    left: 0;
    font-size: 3em;
    font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
    text-align: center;
    letter-spacing: 10px;
    animation: gameover-show 1s ease-in-out forwards;
    color: #e1e1e1;
}

.gameover-overlay .gameover-title {
    display: inline-block;
    width: fit-content;
    position: absolute;
    top: 50%;
    animation: gameover-text-anim 1s ease-out forwards;
}

.gameover-title * {
    filter: drop-shadow(0 10px 10px rgba(0, 0, 0));
}

.gameover-title button {
    cursor: pointer;
    color: inherit;
    font: inherit;
    padding: 8px 16px;
    margin-top: 32px;
    border: none;
    border-radius: 10px;
    transition: letter-spacing 200ms;
    letter-spacing: normal;
    background: linear-gradient(0, #0000, #191919);
} .gameover-title button:hover {
    letter-spacing: 10px;
    animation: button-hover 200ms ease forwards;
}

.gameover-title span {
    display: block;
}

@keyframes gameover-text-anim {
    from {
        opacity: 0;
        transform: translate(-50%, calc(-50% - 100px));
    } to {
        opacity: 100%;
        transform: translate(-50%, -50%);
    }
}


@keyframes gameover-show {
    from {
        background-color: #0000;
    } to {
        background-color: #000000cc;
    }
}