style.css

:root {
  --color-primary: #FFC600;
  --color-secondary: #00A0FF;
  --color-ternary: #5CF2E8;
  --color-quaternary: #F71E35;
}

html {
  font-size: 10px;
  background: url(http://i.imgur.com/b9r5sEL.jpg) bottom center;
  background-size: cover;
}

body, html {
  margin: 0;
  padding: 0;
  font-family: sans-serif;
}

.keys {
  display: flex;
  flex: 1;
  min-height: 100vh;
  align-items: center;
  justify-content: center;
}

.key {
  border: .4rem solid white;
  border-radius: .5rem;
  margin: 1rem;
  font-size: 1.5rem;
  padding: 1rem .5rem;
  transition: all .07s ease;
  width: 10rem;
  text-align: center;
  color: black;
  background: rgba(255, 255, 255, 0.6);
  text-shadow: 0 0 .5rem white;
  cursor: pointer;
}

.playing {
  transform: scale(1.1);
  border-color: var(--color-primary);
  box-shadow: 0 0 1rem var(--color-primary);
  background: var(--color-primary);
  opacity: 0.6;
}

.playing-1 {
  transform: scale(1.1);
  border-color: var(--color-secondary);
  box-shadow: 0 0 1rem var(--color-secondary);
  background: var(--color-secondary);
  opacity: 0.6;
}

.playing-2 {
  transform: scale(1.1);
  border-color: var(--color-ternary);
  box-shadow: 0 0 1rem var(--color-ternary);
  background: var(--color-ternary);
  opacity: 0.6;
}

.playing-3 {
  transform: scale(1.1);
  border-color: var(--color-quaternary);
  box-shadow: 0 0 1rem var(--color-quaternary);
  background: var(--color-quaternary);
  opacity: 0.6;
}

kbd {
  display: block;
  font-size: 4rem;
}

.sound {
  font-size: 1.2rem;
  font-weight: 600;
  text-transform: uppercase;
  letter-spacing: .1rem;
}

.key-repeat {
  width: 11rem;
  margin: 1.4rem;
  font-size: 0;
}

.key-repeat span {
  height: 1.2rem;
  width: 3.66rem;
  display: inline-block;
}

.repeat-1.active {
  background: var(--color-secondary);
}

.repeat-2.active {
  background: var(--color-ternary);
}

.repeat-3.active {
  background: var(--color-quaternary);
}