* {
  padding: 0;
  margin: 0;
  box-sizing: border-box;
}
header {
  height: 65px;
  width: 100%;
  background-color: rgba(34, 49, 52, 0.9);
  /* Change the position property to fixed, and top property to 0 */
  position:fixed;
  top:0px;
  /* Change the z-index property to 10 */
  z-index:100;
}


.header-left {
  float: left;
  padding: 13px;
  font-size: 30px;
  color: rgb(255, 115, 0);
}

.header-right {
  float: right;
  background-color: rgba(255, 255, 255, 0.3);
  transition: all 0.5s;
}

.header-right:hover {
  background-color: rgba(255, 255, 255, 0.5);
}

.header-right a {
  line-height: 65px;
  padding: 0 25px;
  color: white;
  display: block;
}  
body {
  margin-top:65px ;
  padding-top: 20px;
  height: 100px;
  display: flex;
  background: #060AB2;
}

.memory-game {
  width: 500px;
  height: 500px;
  margin: auto;
  display: flex;
  flex-wrap: wrap;
  perspective: 1000px;
}

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

.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: #1C7CCC;
  backface-visibility: hidden;
}

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