An article on shell revealed goes into a great deal of depth about why the Windows Vista Aero Basic theme looks the way it does.
Why does Aero have a solid 1-pixel black border inside the frame, and Aero Basic doesn't? Because the DWM is rendering this black border, and the DWM is absent in Aero Basic. The glass starts in the non-client area and stops after the client glass margin, and there's intentionally no seam between the client and non-client glass areas. This client/non-client blend illusion is kept in Aero Basic for windows without caption text, like Explorer and the Aero Wizards.
Sure, it’s not really that important in the grand scheme of things, but it’s hella interesting from the ‘low-level, nitpicky perspective.’
[This message was pre-recorded; Aaron is currently on vacation]