如何设置Web组件的深色模式？

import web_webview from '@ohos.web.webview';
@Entry
@Component
struct Index {
  controller: web_webview.WebviewController = new web_webview.WebviewController();
  @State mode: WebDarkMode = WebDarkMode.On;//深色模式打开
  build() {
    Column() {
      Web({ src: "https://developer.huawei.com/consumer/cn/", controller: this.controller })
        .darkMode(this.mode)
    }
    .width('100%')
    .height('100%')
    .justifyContent(FlexAlign.Center)
  }
}