The Mac OS user interface is part of the reason Apple is where it is today. Unlike the competition, Apple prided itself on producing a consistent and highly usable UI.
With OS X, part of that has been lost, and we now have a barrage of themes to contend with, including standard Aqua, Unified Aqua, Pro Aqua, Brushed Metal, Polished Metal, and the mess that is the iTunes 7 interface.
UNO aims to bring coherence back, enabling users to unify its interface. Although UNO 1.5 is a complete rewrite, it's still somewhat idiot-proof. The Install toolbar button enables access to a list of appearance options, which are selected via drop-down menus.
Each change updates the useful visual preview, providing an indication of what your Mac's interface will look like once UNO does its thing. Once happy with your selection, a click of the Install button and a restart (or re-login) results in a shiny new interface, based on Apple's Unified Aqua theme.
Actually, 'shiny' isn't entirely accurate - some may find it drab. UNO's main theme is not as light and breezy as Iridium's, for example, but UNO makes up for this in its flexibility (you can skin specific groups of applications), and its installer, which includes a one-click 'restore' function, so you can revert the Mac's appearance to its default state.
And even if UNO's main theme is a bit grey, it's an improvement on Apple's current offering. Safari, iTunes and Finder in particular look great reskinned, and although a few applications don't play nice - like BBEdit - UNO's method of replacing system files rather than 'injecting' applications as they launch means it causes no stability or overhead problems.