如何做到类似web中的整个app变灰

@Entry
@Component
struct Index {
  @State message: string = 'Hello World'

  build() {
    Row() {
      Column() {
        Text(this.message)
          .fontSize(50)
          .fontWeight(FontWeight.Bold)

        Image($r("app.media.icon"))
          .autoResize(true)
          .width(100)
          .height(100)
      }
    }
    .width('100%')
    .height('100%')
    .saturate(0)
  }
}