style.css

* {
  margin: 0;
  padding: 0;
  line-height: 0.5;
}

.body {
  color: #4c5c68;
  font-size: 16px;
}

.container-fluid {
  background-image: url('images/concert.jpg');
  background-repeat: no-repeat;
  background-size: cover;
  background-position: center;
}

.keys {
  display: flex;
  flex-wrap: wrap;
  align-content: center;
  align-items: center;
  justify-content: center;
  min-height: 100vh;
}

.key {
  border: 4px solid black;
  border-radius: 5px;
  margin: 1rem;
  font-size: 1.5rem;
  padding: 1rem 0.5rem;
  transition: all 0.07s;
  width: 100px;
  text-align: center;
  color: white;
  background: rgba(0, 0, 0, 0.4);
  text-shadow: 0 0 5px black;
}

.playing {
  transform: scale(1.1);
  border-color: #ffc600;
  box-shadow: 0 0 10px #ffc600;
}

kbd {
  display: block;
  font-size: 1.5rem;
  font-weight: 200;
  font-family: monospace;
  background-color: rgba(0, 0, 0, 0);
  border-radius: 0;
  padding: 0.2em;
}

.sound {
  font-size: 0.5rem;
  color: #ffc600;
  text-transform: uppercase;
  letter-spacing: 1px;
}