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 (metal or aqua, and already unified windows as well).
On a higher level, UNO's main goal is to enhance aqua interface consistence, by making all elements look and 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.
UNO also fixes some Blue/Graphite appearance inconsistencies.