As shown in the below screenshot, hangout app is fully transparent and it has background-shadow applied to it as well.
Unfortunately you can't currently do this. We have white-listed the hangouts app to be able to use the APIs needed for this.
Opening up these APIs to third parties has the potential for security problems (specifically, phishing and click jacking attacks). Solving these security issues will require a large effort, so we wanted to see if the API was considered useful with a partner app (hangouts) before undertaking that effort.
We're now considering our options and may open the API up to third party apps in the future.