doc/dev/plugins/problems/scss/_accordions.scss

// Accordions
.accordion {
  input:checked ~,
  &[open] {
    & .accordion-header {
      .icon {
        transform: rotate(90deg);
      }
    }

    & .accordion-body {
      max-height: 50rem;
    }
  }

  .accordion-header {
    display: block;
    padding: $unit-1 $unit-2;

    .icon {
      transition: all .2s ease;
    }
  }

  .accordion-body {
    margin-bottom: $layout-spacing;
    max-height: 0;
    overflow: hidden;
    transition: max-height .2s ease;
  }
}

// Remove default details marker in Webkit
summary.accordion-header {
  &::-webkit-details-marker {
    display: none;
  }
}