The modification has been made in Ubuntu 14.04, but it's still good news
Canonical has finally fixed the shutdown dialog for Ubuntu, more precisely the one displayed on the login screen.All Ubuntu distributions suffered from this annoying problem. The window manager is no longer working outside the desktop, and the dialog was shown without a border. There was a time when not even the default theme was applied.
It's a mystery why it took so long for Canonical developers to fix it, but they finally did. The solution is rather elegant and looks the way it should. A Unity-style dialog is now provided, which has replaced the old one.
The bad news is that we will have to wait until the launch of Ubuntu 14.04 LTS (Trusty Tahr) to get it, but that's only six months away. It's unlikely that the new dialog will be added in Ubuntu 13.10 also.