### Preflight Checklist

- [X] I have read the [Contributing Guidelines](https://github.com/electron/electron/blob/main/CONTRIBUTING.md) for this project.
- [X] I agree to follow the [Code of Conduct](https://github.com/electron/electron/blob/main/CODE_OF_CONDUCT.md) that this project adheres to.
- [X] I have searched the [issue tracker](https://www.github.com/electron/electron/issues) for a feature request that matches the one I want to file, without success.

### Electron Version

14.0.1

### What operating system are you using?

macOS

### Operating System Version

macOS Big Sur 11.6.1

### What arch are you using?

x64

### Last Known Working Electron version

14.0.0

### Expected Behavior


When `BrowserView.setBackgroundColor` is called with an explicit color, expect the background to be the color selected.

When `BrowserView.setBackgroundColor` is not called, expect the background of the `BrowserView` to be transparent by default; and can see through `BrowserWindow`'s background color.



### Actual Behavior

The `BrowserView` background is always white.

### Testcase Gist URL

https://gist.github.com/kevinlinv/2de60478ffec6a2f18f1ec0fc6f5add9

### Additional Information

Here is a quick recording that reproduces the error.

https://user-images.githubusercontent.com/44967897/140694174-e27aff28-1847-4813-bc46-d9fda6569128.mp4


shell/browser/api/electron_api_web_contents.cc
Because web preferences don't recognize transparency,
       only set rwhv background color if a color exists
      auto* rwhv = ontents->ontents()->GetRenderWidgetHostView();
      if (rwhv && color.has_value())
        rwhv->SetBackgroundColor(color.value());
    }
  } else {
     Create one if not.
    ontents = WebContents::New(isolate, web_preferences);
  }
  return ontents;
}
 static
WebConbContents::FromID(int32_t id) {
  return GetAllWebContents().Lookup(id);
}
 static
gin::Wo WebContents::kWo = {gin::kEmbedderNativeGin};
e api
e electron
namespace {
using electron::api::GetAllWebContents;
using electron::api::WebContents;
gin::HContents> WebContentsFromID(v8::Isolate* isolate, int32_t id) {
  WebContents* contents = WebContents::FromID(id);
  return contents ? gin::CreateHandle(isolate, contents)
                  : gin::HContents>();
}
gin::HContents> WebContentsFromDevToolsTargetID(
    v8::Isolate* isolate,