可以通过setWindowBackgroundColor实现，示例代码如下：
EntryAbility.ets

export default class EntryAbility extends UIAbility {  
  onWindowStageCreate(windowStage: window.WindowStage): void {  
    AppStorage.setOrCreate("windowStage",windowStage  
    ) //保存windowStage  
  }  
}

Index.ets

import window from '@ohos.window';  
@Entry  
@Component  
struct Index {  
  build() {  
    Row() {  
      Column() {  
        Button('show dialog').onClick(() => {  
          let windowStage_: window.WindowStage = AppStorage.get('windowStage') as window.WindowStage;  
          windowStage_.createSubWindow('subWindow', (err, win) => { //创建透明子窗口并打开  
            win.setUIContent('pages/SubPage');  
            win.showWindow();  
          })  
        })  
      }  
      .width('100%')  
    }  
    .height('100%')  
    .backgroundColor(Color.Pink)  
  }  
}

SubPage.ets

import window from '@ohos.window';  
@Entry  
@Component  
struct SubPage {  
  @State message: string = 'this is a dialog page';  
  
  aboutToAppear() {  
    window.findWindow('subWindow').setWindowBackgroundColor("#00000000") //设置子窗口背景透明  
  }  
  onBackPress() { //关闭子窗口  
    window.findWindow('subWindow').destroyWindow().then((res) => {  
      console.log('destroyWindow success')  
    }).catch(() => {  
      console.log('destroyWindow fail')  
    })  
    return true  
  }  
  build() {  
    Row() {  
      Column() {  
        Text(this.message)  
          .fontSize(16)  
          .fontWeight(FontWeight.Bold)  
          .width(200)  
          .height(150)  
          .backgroundColor(Color.White)  
      }  
      .width('100%')  
    }  
    .backgroundColor("#32ffffff")  
    .height('100%')  
  }  
}