index.css

*{
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

body{
    display: flex;
    justify-content: center;
    align-items: center;
    min-height: 100vh;
    background: #0a3c2f;
}

.container{
    position: relative;
    display: flex;
    justify-content: center;
    align-items: center;
    flex-direction: column;
    gap: 30px;
    background: #0d614b;
    padding: 40px 60px;
}

.container h2{
    margin-left: 30px;
    color: #fff;
    font-family:Cambria;
    font-weight: 800;
    font-size: 2em;
    letter-spacing: 3px;
}

.reset{
    padding: 15px 20px;
    color: #267c65;
    background: #fff;
    border: none;
    font-size: 1.2em;
    text-transform: uppercase;
    cursor: pointer;
    font-weight: 600;
    font-family:Cambria;
    letter-spacing: 2px;
}

.reset:focus{
    color: #fff;
    background: #267c65;
}

.game{
    width: 440px;
    height: 380px;
    display: flex;
    flex-wrap: wrap;
    gap: 10px;
    transform-style: preserve-3d;
    perspective: 500px;
}

.item{
    position: relative;
    width: 100px;
    display: flex;
    justify-content: center;
    align-items: center;
    font-size: 3em;
    background: #fff;
    transform: rotateY(180deg);
    transition: 0.25s;
}

.item.boxOpen{
    transform: rotateY(0deg);
}

.item:after{
    content: '';
    position: absolute;
    inset: 0;
    background: #209d7b;
    transition: 0.25s;
    transform: rotateY(0deg);
    backface-visibility: hidden;
}

.boxOpen:after,
.boxMatch:after{
    transform: rotateY(180deg);
}

.popup{
    position: fixed;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);
    background: #0d614b;
    width: 450px;
    height: 300px;
    box-shadow: 0 25px 50px rgba(0, 0, 0, 0.1),
    0 0 0 1000px rgba(0, 0, 0, 0.90);
    border-radius: 20px;
    display: flex;
    justify-content: center;
    padding: 40px;
    visibility: hidden;
}

.popup h2{
    color: #fff;
    font-family: sans-serif;
    font-size: 35px;
    margin-top: 20px;
}

.popup span{
    margin-left: 80px;
    color: #fff;
}

.active{
    visibility: visible;
}

.close{
    position: absolute;
    top: 0;
    right: 0;
    padding:5px 10px;
    background: #f00;
    border-radius: 0 20px 0 20px;
    color: #fff;
    cursor: pointer;
    font-size: 25px;
}

#my-canvas{
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    height: 100vh;
    z-index: 100000;
    visibility: hidden;
    pointer-events: none;
}

#my-canvas.active{
    visibility: visible;
}