Equinox is an amazing new GTK engine which is already used by the Elementary GTK theme and many others.
Until now, Equinox was only available to download via Gnome Look but starting today, both the Equinox GTK engine and the themes developed by the GTK engine creator (Equinox themes and Equinox Ubuntu themes) got their own PPA so you can stay up to date with the latest version.
(One of the Equinox Ubuntu themes)
To add the Equinox Ubuntu PPA, use the following command in a terminal:
sudo add-apt-repository ppa:tiheum/equinox && sudo apt-get update
Then, to install the Equinox GTK Engine and Equinox-Themes, use the following command:
sudo apt-get install gtk2-engines-equinox equinox-theme