papercss/src/components/_accordion.scss

.collapsible {
  display: flex;
  flex-direction: column;

  &:nth-of-type(1) {
    @include color('border-top-color', 'muted-light');
    border-top-style: solid;
    border-top-width: 1px;
  }

  .collapsible-body {
    @include color('border-bottom-color', 'muted-light');
    @include color('background-color', 'white-dark-light-80');
    @include transition;
    border-bottom-style: solid;
    border-bottom-width: 1px;
    margin: 0;
    max-height: 0;
    opacity: 0;
    overflow: hidden;
    padding: 0 0.75rem;
  }

  > input {
    display: none;

    &:checked + label {
      @include color('color', 'primary');
    }

    &[id^='collapsible']:checked~div.collapsible-body {
      margin: 0;
      max-height: 960px;
      opacity: 1;
      padding: 0.75rem;
    }
  }

  > label {
    @include color('color', 'primary');
    @include color('border-bottom-color', 'muted-light');
    border-bottom-style: solid;
    border-bottom-width: 1px;
    display: inline-block;
    font-weight: 600;
    margin: 0 0 -1px;
    padding: 0.75rem;
    text-align: center;

    &:hover {
      @include color('color', 'muted');
      cursor: pointer;
    }
  }
}