How do I customize the title bar of a popup in a chrome extension?

Question

How do i style the title bar of a popup in a chrome extension? I notice this being done by Chat for Chrome. (Note: the screenshot for that one is outdated)

I got as

Answer 1

I have been researching this topic much and I surely know that Hangouts is a extension, not a packaged app.

I believe the extension windows are modified desktop notifications. I go to create an extension using these modified desktop notifications and i will report the result here.