How to make Cesium background be transparent

I have created 2 CesiumViewer on my pages because I want another 3d overviewmap like Google Earth, and i have configued my CesiumJS like the following codes, to make 3d overviewmap’s background be transparent. but which cannot works

      overviewViewer.scene.highDynamicRange = false
      overviewViewer.scene.globe.enableLighting = false
      overviewViewer.scene.globe.showWaterEffect = false
      overviewViewer.scene.globe.depthTestAgainstTerrain = false = false
      overviewViewer.scene.fog.enabled = false = false = false = false
      overviewViewer.scene.highDynamicRange = false
      overviewViewer.scene.globe.showGroundAtmosphere = false
     //Set the background of the scene to transparent
     overviewViewer.scene.backgroundColor = Cesium.Color.TRANSPARENT
     //HDR needs to be disable for transparent backgrounds
     overviewViewer.scene.highDynamicRange = false

what should i do to remove the black background
Thanks, Levi