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

body {
    height: 100vh;
    display: flex;
    background: #ffffff;
}

#cardboard {
    width: 100%;
    height: 90vh;
    display: flex;
    flex-wrap: wrap;
}

.memory-card {
    width: calc(25% - 5px);
    height: 35%;
    margin: 2px;
    transform-style: preserve-3d;
    transition: 0.3s;
    position: relative;
}

.memory-card:active {
    transform: scale(0.97);
}

.memory-card.flip {
    transform: rotateY(180deg);
}

.front-face,
.back-face {
    width: 100%;
    height: 100%;
    padding: 10px;
    position: absolute;
    background: #675a10;
}

.front-face {
    transform: rotateY(180deg);
}