.progress {
  background: linear-gradient(
    135deg,
    #ff0000,
    #ff1e00,
    #ff7300,
    #fffb00,
    #48ff00,
    #00ffd5,
    #0051ff,
    #002bff,
    #7a00ff,
    #ff00c8,
    #ff0062,
    #ff0000
  );
  background-size: calc(400% + 300px);
  animation: glowing 20s linear infinite;
  transition: opacity 0.3s;
}
.volume-bar {
  position: relative;
}

.glow-on-hover:before,
.volume-bar:before,
.song-menu > button.active-song:before {
  content: "";
  background: linear-gradient(
    135deg,
    #ff0000,
    #ff1e00,
    #ff7300,
    #fffb00,
    #48ff00,
    #00ffd5,
    #0051ff,
    #002bff,
    #7a00ff,
    #ff00c8,
    #ff0062,
    #ff0000
  );
  position: absolute;
  top: -2px;
  left: -2px;
  background-size: 500%;
  z-index: -1;
  filter: blur(5px);
  width: calc(100% + 4px);
  height: calc(100% + 4px);
  animation: glowing 20s linear infinite;
  opacity: 0;
  transition: opacity 0.3s ease-in-out;
  border-radius: 10px;
}

.song-menu > button.active-song:before {
  border-radius: 0;
}

.glow-on-hover:active,
.song-menu > button.active-song:active {
  color: #000;
}

.glow-on-hover:active:after,
.volume-bar:active:after,
.song-menu > button.active-song:after {
  background: transparent;
}

.glow-on-hover:hover:before,
.volume-bar:hover:before,
.song-menu > button.active-song:before {
  opacity: 1;
}

.glow-on-hover:after,
.volume-bar:after,
.song-menu > button.active-song:after {
  z-index: -1;
  content: "";
  position: absolute;
  width: 100%;
  height: 100%;
  background: #111;
  left: 0;
  top: 0;
  border-radius: 10px;
}
.song-menu > button.active-song:after {
  border-radius: 0;
}

@keyframes glowing {
  0% {
    background-position: 0 0;
  }
  50% {
    background-position: 400% 0;
  }
  100% {
    background-position: 0 0;
  }
}