style.css

/* The cards will appear after createGameBoard() function in js. */
.grid {
  display: grid;
  grid-template-columns: repeat(4, 100px);
  gap: 10px;
  justify-content: center;
  margin-top: 20px;
}

.card {
  width: 100px;
  height: 100px;
  background-color: #3498db;
  color: #fff;
  display: flex;
  justify-content: center;
  align-items: center;
  cursor: pointer;
  border-radius: 5px;
  -webkit-border-radius: 5px;
  -moz-border-radius: 5px;
  -ms-border-radius: 5px;
  -o-border-radius: 5px;
  transform-style: preserve-3d;
  transition: all 0.5s ease-in-out;
  -webkit-transition: all 0.5s ease-in-out;
  -moz-transition: all 0.5s ease-in-out;
  -ms-transition: all 0.5s ease-in-out;
  -o-transition: all 0.5s ease-in-out;
}

.card.face-up {
  transform: rotate(360deg) !important;
  -webkit-transform: rotate(360deg) !important;
  -moz-transform: rotate(360deg) !important;
  -ms-transform: rotate(360deg) !important;
  -o-transform: rotate(360deg) !important;
  background-color: #2ecc71;
  cursor: default;
}

.card.face-down {
  transform: rotate(0deg);
  -webkit-transform: rotate(0deg);
  -moz-transform: rotate(0deg);
  -ms-transform: rotate(0deg);
  -o-transform: rotate(0deg);
}

.card.match {
  background-color: rgb(250, 54, 87);
}

header {
  text-align: center;
  margin-bottom: 20px;
}

#restart-button {
  padding: 10px 20px;
  background-color: #e74c3c;
  color: #fff;
  border: none;
  border-radius: 5px;
  -webkit-border-radius: 5px;
  -moz-border-radius: 5px;
  -ms-border-radius: 5px;
  -o-border-radius: 5px;
  cursor: pointer;
  font-size: 18px;
  transition: background-color 0.3s ease;
  -webkit-transition: background-color 0.3s ease;
  -moz-transition: background-color 0.3s ease;
  -ms-transition: background-color 0.3s ease;
  -o-transition: background-color 0.3s ease;
}

#restart-button:hover {
  background-color: #c0392b;
}