Apple's design changes over the years have been much more subtle, mostly changing textures, fonts, line widths and control outlines. Meanwhile they've kept all the software on the same UI toolkit so they can update these things universally.

This means that even the little-seen parts of the OS that have never been redesigned (like DigitalColor Meter, the Bluetooth file receive dialog, ColorSync Utility, Keychain Assistant) still look pretty much at-home in the latest design.