Bootstrap/scss/_accordion.scss

//
// Base styles
//

.accordion-button {
  position: relative;
  display: flex;
  align-items: center;
  width: 100%;
  padding: $accordion-button-padding-y $accordion-button-padding-x;
  @include font-size($font-size-base);
  color: $accordion-button-color;
  text-align: left; // Reset button style
  background-color: $accordion-button-bg;
  border: $accordion-border-width solid $accordion-border-color;
  @include border-radius(0);
  overflow-anchor: none;
  @include transition($accordion-transition);

  &.collapsed {
    border-bottom-width: 0;
  }

  &:not(.collapsed) {
    color: $accordion-button-active-color;
    background-color: $accordion-button-active-bg;

    &::after {
      background-image: escape-svg($accordion-button-active-icon);
      transform: $accordion-icon-transform;
    }
  }

  // Accordion icon
  &::after {
    flex-shrink: 0;
    width: $accordion-icon-width;
    height: $accordion-icon-width;
    margin-left: auto;
    content: "";
    background-image: escape-svg($accordion-button-icon);
    background-repeat: no-repeat;
    background-size: $accordion-icon-width;
    @include transition($accordion-icon-transition);
  }

  &:hover {
    z-index: 2;
  }

  &:focus {
    z-index: 3;
    border-color: $accordion-button-focus-border-color;
    outline: 0;
    box-shadow: $accordion-button-focus-box-shadow;
  }
}

.accordion-header {
  margin-bottom: 0;
}

.accordion-item {
  &:first-of-type {
    .accordion-button {
      @include border-top-radius($accordion-border-radius);
    }
  }

  &:last-of-type {
    .accordion-button {
      // Only set a border-radius on the last item if the accordion is collapsed
      &.collapsed {
        border-bottom-width: $accordion-border-width;
        @include border-bottom-radius($accordion-border-radius);
      }
    }

    .accordion-collapse {
      border-bottom-width: $accordion-border-width;
      @include border-bottom-radius($accordion-border-radius);
    }
  }
}

.accordion-collapse {
  border: solid $accordion-border-color;
  border-width: 0 $accordion-border-width;
}

.accordion-body {
  padding: $accordion-body-padding-y $accordion-body-padding-x;
}


// Flush accordion items
//
// Remove borders and border-radius to keep accordion items edge-to-edge.

.accordion-flush {
  .accordion-button {
    border-right: 0;
    border-left: 0;
    @include border-radius(0);
  }

  .accordion-collapse {
    border-width: 0;
  }

  .accordion-item {
    &:first-of-type {
      .accordion-button {
        border-top-width: 0;
        @include border-top-radius(0);
      }
    }

    &:last-of-type {
      .accordion-button.collapsed {
        border-bottom-width: 0;
        @include border-bottom-radius(0);
      }
    }
  }
}