Orion is a new, clear GTK3 theme created by Satya, who's also behind other beautiful GTK and GNOME Shell / Cinnamon themes like Evolve, Shine, Minty, Ambiance Blue and others.
The GTK3 theme is powered by the Unico GTK3 engine, comes with a modified Boomerang Metacity theme and is compatible with GTK 3.2 (both GNOME Shell and Unity). Orion also includes a GTK2 theme, for compatibility with GTK2 applications.
As usual, Satya has uploaded the theme to his themes PPA so if you use Ubuntu 11.10 Oneiric Ocelot, you can install it using the commands below:
sudo apt-add-repository ppa:satyajit-happy/themes sudo apt-get update sudo apt-get install orion-gtk-theme
For other Linux distributions, download Orion via DeviantArt.
Note: the theme is still work in progress.
Thanks to Satya for the tip!