In this page, we will introduce 15 best GTK3 themes that you can use with Ubuntu 12.04 (Precise Pangolin) under Unity or Gnome Shell. The themes are also compatible with other Ubuntu versions (12.10/11.10, Linux Mint 13, etc.). To make the installation of these themes easier, I have uploaded them all to our custom PPA. You can add our PPA with these commands:
sudo add-apt-repository ppa:upubuntu-com/gtk3
sudo apt-get update
To enable these themes, you can use GNOME TWEAK TOOL, MyUnity, or use simply the commands provided with each theme given below. Gnome Tweak Too can be installed with this command:
sudo apt-get install gnome-tweak-tool

1. Elementary GTK Theme

sudo apt-get install elementary
gsettings set org.gnome.desktop.interface gtk-theme 'elementary'
gconftool-2 --set --type string /apps/metacity/general/theme 'elementary'
2. Zukitwo

sudo apt-get install zukitwo
gsettings set org.gnome.desktop.interface gtk-theme 'zukitwo'
gconftool-2 --set --type string /apps/metacity/general/theme 'zukitwo'
3. SLAVE GTK3 Theme

sudo apt-get install slave-gtk3
gsettings set org.gnome.desktop.interface gtk-theme 'SLAVE-GTK3'
gconftool-2 --set --type string /apps/metacity/general/theme 'SLAVE-GTK3'
4. Boomerang

sudo apt-get install boomerang
gsettings set org.gnome.desktop.interface gtk-theme 'Boomerang'
gconftool-2 --set --type string /apps/metacity/general/theme 'Boomerang'
For Boomerang-Deux:
gsettings set org.gnome.desktop.interface gtk-theme 'Boomerang-Deux'
gconftool-2 --set --type string /apps/metacity/general/theme 'Boomerang-Deux'
5. Zukiwi

sudo apt-get install zukiwi
gsettings set org.gnome.desktop.interface gtk-theme 'Zukiwi'
gconftool-2 --set --type string /apps/metacity/general/theme 'Zukiwi'
6. Elegant Brit GNOME3

sudo apt-get install elegant-brit-theme
gsettings set org.gnome.desktop.interface gtk-theme 'Elegant_Brit'
gconftool-2 --set --type string /apps/metacity/general/theme 'Elegant_Brit'
7. GnomishGray

sudo apt-get install gnomishgray
gsettings set org.gnome.desktop.interface gtk-theme 'GnomishGray'
gconftool-2 --set --type string /apps/metacity/general/theme 'GnomishGray'
8. Drakfire Zuki

sudo apt-get install drakfire-zuki
gsettings set org.gnome.desktop.interface gtk-theme 'Drakfire-Zuki'
gconftool-2 --set --type string /apps/metacity/general/theme 'Drakfire-Zuki'
9. DeLorean_Noir

sudo apt-get install delorean-noir-theme
gsettings set org.gnome.desktop.interface gtk-theme 'DeLorean_Noir'
gconftool-2 --set --type string /apps/metacity/general/theme 'DeLorean_Noirs'
10. Yellow and Black Attack

sudo apt-get install yellow-n-black-attack
gsettings set org.gnome.desktop.interface gtk-theme 'Yellow-n-Black-Attack-GTK'
gconftool-2 --set --type string /apps/metacity/general/theme 'Yellow-n-Black-Attack-GTK'
11. Ice Cream GTK

sudo apt-get install ice-cream-theme
gsettings set org.gnome.desktop.interface gtk-theme 'Ice-Cream-GTK'
gconftool-2 --set --type string /apps/metacity/general/theme 'Ice-Cream-GTK'
12. Light-Greyness-Dark-Grey-GTK

sudo apt-get install light-greyness-dark-grey
gsettings set org.gnome.desktop.interface gtk-theme 'Light-Greyness-Dark-Grey-GTK'
gconftool-2 --set --type string /apps/metacity/general/theme 'Light-Greyness-Dark-Grey-GTK'
13. ICE Redux

sudo apt-get install ice-redux
gsettings set org.gnome.desktop.interface gtk-theme 'ICE-Redux'
gconftool-2 --set --type string /apps/metacity/general/theme 'ICE-Redux'
14. Oceanic Dark

sudo apt-get install oceanic-dark
gsettings set org.gnome.desktop.interface gtk-theme 'Oceanic-Dark'
gconftool-2 --set --type string /apps/metacity/general/theme 'Oceanic-Dark'
15. Mac OS X Lion Theme

sudo apt-get install macx-lion
gsettings set org.gnome.desktop.interface gtk-theme 'Mac-X-Lion'
gconftool-2 --set --type string /apps/metacity/general/theme 'Mac-X-Lion'
You can make these themes more attractive by using an icon theme from this list.