伪元素的写法

  .el-table__header .el-table-column--selection .cell .el-checkbox:hover:after {
    content: "全选";
    border-color: #e5e5e5;
    background-color: #f1f1f1;
    margin-left: 6px;
    font-size: 12px;
    padding: 6px;
  }