.checkbox-mp3 input {
  position: absolute;
  opacity: 0;
  cursor: pointer;
  height: 0;
  width: 0;
}

.checkbox-mp3 {
  display: block;
  position: relative;
  cursor: pointer;
  font-size: 1.5rem;
  user-select: none;
}

.checkmark {
  --clr: var(--checkbox-bg-checked);
  position: relative;
  top: 0;
  left: 0;
  height: 1.3em;
  width: 1.3em;
  background-color: var(--checkbox-bg-default);
  border-radius: .5rem;
  transition: background-color 300ms, border-radius 300ms;
}

.checkbox-mp3 input:checked ~ .checkmark {
  background-color: var(--clr);
  border-radius: .5rem;
  animation: pulse 500ms ease-in-out;
}

.checkmark:after {
  content: "";
  position: absolute;
  display: none;
}

.checkbox-mp3 input:checked ~ .checkmark:after {
  display: block;
}

.container .checkmark:after {
  left: 0.45em;
  top: 0.25em;
  width: 0.25em;
  height: 0.5em;
  border: solid var(--checkbox-checkmark-border-color);
  border-width: 0 0.15em 0.15em 0;
  transform: rotate(45deg);
}

@keyframes pulse {
  0% {
    box-shadow: 0 0 0 var(--checkbox-pulse-color);
    rotate: 20deg;
  }

  50% {
    rotate: -20deg;
  }

  75% {
    box-shadow: 0 0 0 10px var(--checkbox-pulse-color);
  }

  100% {
    box-shadow: 0 0 0 13px var(--checkbox-pulse-color);
    rotate: 0;
  }
}