ant-design/style/components/collapse.less

@collapse-prefix-cls: ant-collapse;

.collapse-close() {
  .iconfont-size-under-12px(7px, 270deg);
}
.collapse-open() {
  .iconfont-size-under-12px(7px, 360deg);
}

.@{collapse-prefix-cls} {
  background-color: #f4f4f4;
  border-radius: 3px;
  border: 1px solid @border-color-base;

  & > &-item {
    border-top: 1px solid @border-color-base;
    &:first-child {
      border-top: 0;
    }

    > .@{collapse-prefix-cls}-header {
      height: 38px;
      line-height: 38px;
      padding-left: 32px;
      color: #666;
      cursor: pointer;
      position: relative;

      .arrow {
        .collapse-close();
        .iconfont-mixin();
        position: absolute;
        color: #666;
        display: inline-block;
        margin-right: 8px;
        line-height: 40px;
        content: "\e611";
        vertical-align: middle;
        transition: transform 0.24s ease;
        top: 0;
        left: 16px;
        top: ~"16px \9";
        left: ~"0 \9";
        &:before {
          content: "\e611";
        }
      }
    }
  }

  &-content {
    overflow: hidden;
    color: @text-color;
    padding: 0 16px;
    background-color: #fff;
    transition: height .2s @ease-out;

    & > &-box {
      padding-top: 16px;
      padding-bottom: 16px;
    }

    &-inactive {
      display: none;
    }
  }

  &-item:last-child {
    > .@{collapse-prefix-cls}-content {
      border-radius: 0 0 3px 3px;
    }
  }

  & > &-item > &-header[aria-expanded="true"] {
    .arrow {
      .collapse-open();
    }
  }
}