Download a theme>extract it>Inside it should have something similar to this:
As you can see there's a gtk-3.0 and a metacity-1 folder. These two folders should be in: /home/user/.themes/.
Then open up Gnome Tweak Tool>go to Theme>Window theme>Scroll down to the theme you want.
Log out, and done.
Today after installing Gnome Shell from the Software Center in Ubuntu 12.10 my Ambience and Radiance themes got corrupted as well. I found this post on the Ubuntu Forums which lead me to the solution for the problem.
The problem seems to be that after installing Gnome Shell the default theme is Adwaita (in gnome-tweak-tool). It tells me that Adwaita is the default and before it was Ambiance. And because of this when I select the theme Ambiance some components of Adwaita persist, especially selected items in the menu bar in Gnome Classic appear as gray text on gray background.
I noticed that this, in contrast to what nils8950 said, only happens with GTK2 applications, not with GTK3 applications.
So after reading that Ubuntu Forum post, I looked at the contents of the file ~/.gtkrc-2.0 which is a hidden file in your home directory. The contents of this file looked as follows:
# -- THEME AUTO-WRITTEN BY gtk-theme-switch2 DO NOT EDIT
include "/usr/share/themes/Adwaita/gtk-2.0/gtkrc"
include "/home/serrano/.gtkrc-2.0.mine"
# -- THEME AUTO-WRITTEN BY gtk-theme-switch2 DO NOT EDIT
Which explains the problem. You can see that it loads a theme file for the Adwaita theme. You could simply remove the line that reads include "/usr/share/themes/Adwaita/gtk-2.0/gtkrc" to solve this problem, though the file clearly states DO NOT EDIT. You can see that this file was auto-written by a tool named gtk-theme-switch2. As an alternative, we can simply use that tool to change the theme back to Ambiance. If you don't already have it installed:
sudo apt-get install gtk-theme-switch
Then execute the tool from the terminal:
gtk-theme-switch2
Select Ambiance from the dropdown menu and press the Apply button. Make sure Ambiance is also selected in gnome-tweak-tool. You may need to restart some applications to see the changes (no need to reboot the system).
Best Answer
the problem is that gnome 3.4 totally changed the way themes are written, which means, gtk3.2 themes (including this one) wont work on gnome3.4
one way is to downgrade to gnome3.2 from 3.4
there are 'a few' (very few) themes ported to gnome 3.4
and as far as i know theme developers are not so happy about this too, i guess, talked to some of them, they aren't happy!