Office-UI-Fabric-React:如何替换CommandBar菜单中的"ChevronDown“和"ChevronRight”图标？

import { CommandBar } from 'office-ui-fabric-react';
import { ChevronDownIcon, ChevronRightIcon } from '@fluentui/react-icons-mdl2';

const onRenderChevronDown = () => {
  return <ChevronDownIcon />;
};

const onRenderChevronRight = () => {
  return <ChevronRightIcon />;
};

<CommandBar
  items={[
    {
      key: 'item1',
      name: 'Item 1',
      iconProps: {
        iconName: 'ChevronDown',
        onRenderIcon: onRenderChevronDown,
      },
    },
    {
      key: 'item2',
      name: 'Item 2',
      iconProps: {
        iconName: 'ChevronRight',
        onRenderIcon: onRenderChevronRight,
      },
    },
  ]}
/>