Satya who designed many cool GTK3 and Gnome Shell themes has recently released/updated 3 new themes - Gaia, Elementary Dark and Orion.
You can easily install these themes from a PPA. To add the PPA, run these commands and then follow instructions below.
sudo apt-get update
sudo apt-get install gnome-shell-extensions-user-theme
Gnome Shell - Gaia
Gaia Shell is clean and beautiful theme for Gnome Shell inspired by the Gaia project. It tries to represent the colors of nature while maintaining a clean and aesthetic look. Gaia Shell is compatible with the latest Gnome Shell 3.2.
Install by running the command: sudo apt-get install gnome-shell-theme-gaia
Elementary Dark GTK3 Theme
Elementary Dark is a soft and subtle dark theme based on the higly popular elementary theme by DanRabbit.
Install by running the command: sudo apt-get install elementary-dark-theme
Orion GTK3 Theme
Orion is a modern light theme for Gnome. The theme is compatible with the latest GTK 3.2 and uses the Unico engine.
Install by running the command: sudo apt-get install orion-gtk-theme