element/packages/theme-chalk/src/collapse.scss

@import "mixins/mixins";
@import "common/var";

@include b(collapse) {
  border-top: 1px solid $--collapse-border-color;
  border-bottom: 1px solid $--collapse-border-color;
}
@include b(collapse-item) {
  @include e(header) {
    height: $--collapse-header-height;
    line-height: $--collapse-header-height;
    background-color: $--collapse-header-fill;
    color: $--collapse-header-color;
    cursor: pointer;
    border-bottom: 1px solid $--collapse-border-color;
    font-size: $--collapse-header-size;
    font-weight: 500;
    transition: border-bottom-color .3s;
    &:focus:not(.focusing), &:active {
      outline-width: 0;
    }

    @include e(arrow) {
      margin-right: 8px;
      transition: transform .3s;
      float: right;
      line-height: 48px;
      font-weight: 300;
    }
  }

  @include e(wrap) {
    will-change: height;
    background-color: $--collapse-content-fill;
    overflow: hidden;
    box-sizing: border-box;
    border-bottom: 1px solid $--collapse-border-color;
  }

  @include e(content) {
    padding-bottom: 25px;
    font-size: $--collapse-content-size;
    color: $--collapse-content-color;
    line-height: 1.769230769230769;
  }

  @include when(active) {
    .el-collapse-item__header {
      border-bottom-color: transparent;
      .el-collapse-item__arrow {
        transform: rotate(90deg);
      }
    }
  }

  &:last-child {
    margin-bottom: -1px;
  }
}