Gstyle is a full gnome theme manager.It can manage and install Gtk themes,Icons themes,Wallpapers/Walltimes,Window decoration,gdm,and so on.
All themes downloaded from gnome-look,deviantart, customize.org can be easily installed with gstyle,and gstyle itself provides full-pack themes downloading.

Install Gstyle:
For Ubuntu 9.10 and higher,open terminal from Applications->Accessories menu and execute following commands:

sudo apt-add-repository ppa:s-lagui
sudo apt-get update
sudo apt-get install gstyle

For Ubuntu 9.04 and older:
edit /etc/apt/sources.list file by this command

sudo gedit /etc/apt/sources.list

add following lines into the end and save the file.

deb http://ppa.launchpad.net/s-lagui/ppa/ubuntu jaunty main 
deb-src http://ppa.launchpad.net/s-lagui/ppa/ubuntu jaunty main 

Remember replace “jaunty” with yours Ubuntu version name.
Now, add gpg key,update system and install gstyle:

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys C3BB95BB 
sudo apt-get update
sudo apt-get install gstyle

Gsytle can be launched from System->Preferences menu.

Fix Gstyle doesn’t launch
Some encounter key mismatch error when launching Gsyte although it works well in my Ubuntu 10.04.
Type gstyle in terminal to launch and get the error,then try following to fox:

Press Alt+F2 and type gconf-editor in pop-up window, in next window navigate to desktop/gnome/interfaces.Right-click in right box to create a new key.In Name area type the key name in the error report,and set its type to “String”