style.css

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

:root{
    --bg-main:#464646;
    --bg-card:#2D2727;
    --border-color:#efc745;
}

body{
    height: 100vh;
    display: flex;
    justify-content: center;
    align-items: center;
    flex-direction: column;
    gap: 5rem;
    background-color: var(--bg-main);
}

h1{
    color: #fff;
    font-family: Arial, Helvetica, sans-serif;
    font-size: 4rem;
}

span{
    color: var(--border-color);
}

#card-section{
    display: grid;
    grid-template-columns: repeat(4,1fr);
    grid-gap: 2rem;
}

.card{
    position: relative;
    transition: all .4s linear;
    transform-style: preserve-3d;
}

.card,
.back-card,
.front-card{
    height: 10rem;
    width: 10rem;
}

.back-card, 
.front-card{
    position: absolute;
    backface-visibility: hidden;

}

.front-card{
    background: var(--border-color);
    background-size: contain;
    border-radius: .6rem;
    box-shadow: rgba(149, 157, 165, 0.2) 0px 8px 24px;
    padding: 2rem;
}

.back-card{
    transform: rotateY(180deg);
    background-size: contain;
    background-repeat: no-repeat;
    background-position: center center;
    box-shadow: rgba(149, 157, 164, 0.2) 0px 8px 24px;
    padding: 2rem;
}


.card_selected{
    transform: rotateY(180deg);
}

.card_match .front-card{
    background-color: var(--bg-card);
}