style.css

*,
*::before,
*::after {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}
/*Root declaration*/
:root {
  --white: #fff;
  --yellow: #ffc600;
}
body {
  font-size: 10px;
  background: url('./images/OIP.webp') bottom center;
  background-size: cover;
  font-family: sans-serif;
}
.main {
  display: flex;
  flex-flow: column nowrap;
  justify-content: center;
  align-items: center;
  min-height: 100vh;
  overflow: auto;
}
.card {
  display: flex;
  flex-direction: column;
  width: 100%;
  text-align: center;
}
.keys {
  display: flex;
  flex-flow: row wrap;
  gap: 1rem;
  align-items: center;
  justify-content: center;
  cursor: pointer;
}
.key {
  border: .4rem solid black;
  border-radius: 1rem;
  font-size: 1.5rem;
  padding: 1rem 0;
  transition: all 0.7s ease;
  text-align: center;
  color: var(--white);
  background: rgba(0,0,0,0.4);
  text-shadow: 0 0 .5rem black;
}
.test {
  width: 8rem;
}
.playing {
  transform: scale(1.1);
  border-color: var(--yellow);
  box-shadow: 0 0 1rem var(--yellow);
  color: var(--yellow);
}
kbd {
  display: block;
  font-size: 4rem;
}
.sound {
  font-size: 1.2rem;
  text-transform: uppercase;
  letter-spacing: .1rem;
  color: var(--yellow);
}
h1, .custom {
  color: var(--white);
  margin-bottom: 1rem;
  padding: 0.5em;
  font-size: 1.5rem;
  text-transform: uppercase;
  display: inline-block;
  border: none;
  border-bottom: 0.5em solid var(--yellow);
  text-shadow: 0 0 .5rem black;
}
.custom  {
  border: 0.5em solid var(--yellow);
  margin-top: 2rem;
  cursor: pointer;
  transition: all 5s ease;
  display: none;
}
.heading--mobile {
  display: none;
}
@media (max-width: 685px) {
  .heading--desktop{
    display: none;
  }
  .heading--mobile {
    display: block;
  }
}