Pro Theme is a GUI style used by Apple Inc. for its Mac OS X operating system. It is reserved for Apple's high-end video, music and image editing products, such as Final Cut Pro HD, Soundtrack Pro and Aperture.
The most notable difference between Pro and Apple's other GUI styles is the abandonment of white and vibrantly coloured icons. Pro applications also use a much smaller typeface than the other themes.
The Pro theme is based on a private Framework called ProKit.framework, which is stored in the computer's /System/Library/PrivateFrameworks/ folder. It is not available for use in non-Apple applications.
References
Pro theme Wikipedia(Text) CC BY-SA