How do I change to a theme not listed in the Appearance screen?

It's not a problem of where they are located. It is a matter of how 11.10 and GNOME 3 have changed. For some reason (I don't know why) the GNOME developers have removed appearance properties from the system settings, under the impression that users should not have the power to choose their own theme.

The Ubuntu developers where kind enough to patch the control centre to allow for us to use Ambiance and Radiance, but if you want to use other GTK+ 3 themes you will need to install gnome-tweak-tool . After installing, you can find it by opening "Advanced Settings" from the Dash.

13.04 and later

Themes can be changed through the excellent Unity customisation tool called unity-tweak-tool

11.04/11.10/12.04

(click to install)

Themes can now be changed through the excellent Unity customisation tool called MyUnity. The key advantage over using Gnome-Tweak is that it does not install all the gnome-shell dependencies.

Changing to any installed theme is as simple as clicking on the theme as shown in the list. You can similarly change the icon set via the right-hand list on the same screen.

to install

for 12.04 - MyUnity

for 11.04/11.10:

sudo add-apt-repository ppa:myunity/ppa
sudo apt-get update
sudo apt-get install myunity