style.css

* {
  box-sizing: border-box;
}
body {
  margin: 0;
  padding: 0 1em;
  background-color: #161B1F;
  font-size: 18px;
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif, "Apple Color Emoji", "Segoe UI Emoji", "Segoe UI Symbol";
}
a {
  color: yellow;
}
.header {
  text-align: center;
  margin: 3em 0;
  color: yellow;
}
.keys {
  display: flex;
  flex-wrap: wrap;
  justify-content: space-around;
  max-width: 64em;
  margin: 3em auto;
  padding: 1em;
  border: 3px solid #EEEEEE;
  border-radius: 1em;
}
.key {
  width: 100%;
  padding: 1em;
}
.key--playing .key__content {
  background: yellow!important;
}
.key__content {
  position: relative;
  height: 200px;
  padding: .75em;
  transition: all .1s;
  border-radius: 1em;
  background: #999999;
}
.key__content span {
  display: block;
  width: 100%;
  height: 100%;
  color: #FFFFFF;
  padding: .75em;
  text-transform: uppercase;
  font-weight: bold;
  background-color: #161B1F;
}

@media screen and (min-width: 35.8em) {
  .key {
    width: 33.3333%;
  }
  .key__content {
    height: 300px;
  }
}