An update brought, weeks ago, border-less Ambiance&Radiance in Natty Narwhal, and, shortly after, the developers changed back to bordered Ambiance&Radiance.
Yes, but, why?
There were problem with Unity 2D (Unity 3D's less-3D brother) and border-less themes.
All was great in Unity 3D, and, the developers proposed a "trick" :the testers/users should continue to use bordered themes, and, when the devs can fix the Unity-2D-related bug, the border-less themes to be reintroduced.
At the moment, the problem still hasn't been fixed.