UNO derives from the Latin word for one and stands for "as one". UNO is the root of Unity.

UNO is a theme that brings the sunken unified toolbar/titlebar look&feel to every single window on your system (cocoa or carbon, metal or aqua and already unified windows as well).
On an higher level, UNO's main goal is to enhance aqua interface consistence, by making all elements look&feel "as one".
UNO is aimed to those who want a clean and un-osbstructive interface while keeping the best of Aqua.
The unity level proposed by UNO does not compromises overall GUI contrast: UNO and UNO shade can be mixed up in order to achieve that usability goal.