style.css

body {
  /*background-image: url("https://audio.andyp4245.repl.co/bg.jpg");*/
  background-color: skyblue;
  font-family: system-ui;
  font-color: black;
  /*text-align: center;*/
  padding: 120px;
}

h1 {
  font-family: cursive;
  text-align: center;
  font-size: 2.5em;
}


#song_list {
  background: white;
  border-radius: 10px;
}


#play-btn {
  position: relative;
  padding: 2.5rem;
  text-align: center;
  display: flex;
  justify-content: center;
  align-items: center;
  overflow: hidden;
  border-radius: 10px;
  color: white;
  font-size: 2.5em;
  z-index: 2;
}

#play-btn::before {
  content: "";
  position: absolute;
  z-index: -1;
  top: 0;
  left: 0;
  width: 300%;
  height: 100%;
  background-image: linear-gradient(
    145deg,
    #f60000,
    #cd00cd,
    #ff8c00,
    #ffee00,
    #4de94c,
    #f60000,
    #cd00cd,
    #ff8c00,
    #ffee00,
    #4de94c,
    #f60000,
    #cd00cd
  );
  border-radius: 10px;
  animation: animate 1.5s linear infinite;
}

@keyframes animate {
  to {
    transform: translateX(-50%);
  }
}

#play-btn::after {
  content: "";
  z-index: -1;
  background: black;
  position: absolute;
  width: calc(100% - 1rem);
  height: calc(100% - 1rem);
  border-radius: 10px;
}