style.css

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

body {
  height: 100vh;
  display: flex;
  /* background-color: aqua; */
}

.memory-game {
  width: 640px;
  height: 640px;
  margin: auto;
  display: flex;
  flex-wrap: wrap;
  /* border: 2px solid blue; */
  perspective: 1000px;
}

.memory-card {
  width: calc(25% - 10px);
  height: calc(33.33% - 10px);
  margin: 5px;
  position: relative;
  border: 2px purple;
  transform: scale(1);
  transform-style: preserve-3d;
  transition: transform 0.5s;
}

.memory-card:active {
  transform: scale(0.97);
  transition: transform 0.2s;
}

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

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

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

.welcome {
  background-color: #f15b31;
  color: #ffffff;
  padding: 30px 120px 30px 120px;
  text-align: center;
  margin-bottom: 1.5rem;
}

.instructions {
  background-color: #00a6b3;
  color: #ffffff;
  /* border-top: 2em solid white;
  border-bottom: 4px solid white;
  border-left: 15em solid white;
  border-right: 15em solid white; */
  padding-top: 20px;
  padding-bottom: 20px;
  margin-bottom: 1.5rem;
}

h2 {
  text-align: center;
}

h4{
  margin: 1.5rem;
  text-indent: 1.5rem;
}