style.css

html {
  background-color: #000000;
  font-size: 10rem;
  box-sizing: border-box;
  }
  .container {
    margin-top: 12vh;
    display: flex;
    justify-content: center;
  }
  
  .card {
    width: 300px;
    height: 400px;
    perspective: 600px;
    background-color: transparent;
  }
  
  .card-inner {
    position: relative;
    width: 100%;
    height: 100%;
    text-align: center;
    animation: rotate 5s linear forwards infinite;
    transform-style: preserve-3d;
  }
  
  .card-fs,
  .card-bs {
    width: 100%;
    height: 100%;
    position: absolute;
    backface-visibility: hidden;
  }
  
  .card-fs {
    background-color: rgb(147, 134, 218);
  }
  
  .card-bs {
    background-color: rgb(33, 224, 147);
    transform: rotateY(180deg);
  }
  

  @keyframes rotate {
    from {
      transform: rotateY(0deg);
    }
    to {
      transform: rotateY(360deg);
    }
  }