Orion is a light GTK3 theme compatible with GTK 3.2, it uses Unico theming engine. Also, it comes with GTK2 theme, metacity, Unity and Orion Xfce theme. Orion theme designed by “satya164”. Check this installation instruction for Orion GTK3 theme on Ubuntu 11.10 Oneiric Ocelot and Ubuntu 12.04 LTS Precise Pangolin Using PPA.
Install Orion Theme on Ubuntu 11.10 Oneiric Ocelot and Ubuntu 12.04 LTS Precise Pangolin Using PPA
Run Terminal “Ctrl+Alt+t”
sudo apt-add-repository ppa:satyajit-happy/themes
sudo apt-get update
sudo apt-get install orion-gtk-theme
Go to Gnome tweak tool, if you don’t have, install it using this command
sudo apt-get install gnome-tweak-tool
From “Theme”, select the “Orion” from the drop down menu for “GTK+ theme” and “Window Theme” as well.
For non Ubuntu users
You can Download Orion GTK3 theme package and extract it to the theme directory.
and use the same instructions using Gnome-Tweak-Tool mentioned up here to activate Orion Theme.
Source Deviantart Orion GTK3 Theme.