el-dialog title下加样式

 <el-dialog
      :title="title"
      :visible.sync="open"
      width="500px"
      append-to-body
      :close-on-click-modal="false"
      class="alertlablefixtitle"
    >
    .alertlablefixtitle .el-dialog__header {
  border-bottom: gainsboro 1px solid;
}