styles.css

.game {
  margin: 50px auto;
  width: 44%;
  min-width: 300px;
}

@media screen and (max-width: 768px) {
  .game {
    width: 66%;
  }
}

.game-Board {
  margin: 0;
  padding: 0;
  display: block;
}

.memory-card {
  position: relative;
  width: 23%;
  height: 23%;
  padding: 0;
  margin: 1%;
  box-sizing: border-box;
  display: inline-block;
  white-space: normal;
  color:black;
  background-color: grey;
  border-radius: 4px;
  border: 8px solid white;
  box-shadow: 1px 1px 4px grey;
  background-size: 150%;
}

.memory-card--mismatch {
  border-color: red;
}

.memory-card:not(.found) {
  background-image: url("media/contemporary_china.png");
}

.memory-card .memory-card__content {
  position: absolute;
  top: 0;
  right: 0;
  bottom: 0;
  left: 0;
  text-align: center;
  display: inline-block;
  background-size: cover;
}

.centered {
  text-align: center;
}

.square {
  position: relative;
  width: 100%;
}

.square:after {
  content: "";
  display: block;
  padding-bottom: 100%;
}

.square__content {
  position: absolute;
  width: 100%;
  height: 100%;
}