index.css

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

body{
    display: flex;
    height: 100vh;
    background: #B100FE;
}

.memory-game{
    margin: auto;
    width: 640px ;
    height: 640px;
    display: flex ;
    flex-wrap: wrap;
    perspective: 1000px;
}

.memory-card {
    width: calc(25% - 10px);
    height: calc(33.333% - 10px);
    margin: 5px;
    position: relative;
    box-shadow: 1px 1px 1px rgba(0,0,0,.3);
    transform: scale(1);
    transform-style: preserve-3d;
    transition: transform .5s;
}

.memory-card:active{
    transform: scale(0.97);
    transition: transform .2s;
}
.memory-card.flip{
    transform: rotateY(180deg);
}



.front-face,
.back-face {
  width: 100%;
  height: 100%;
  padding: 20px;
  position: absolute;
  border-radius: 5px;
  background: #D76EFE;
  backface-visibility: hidden;
}

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