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.

What themes you can use with Unity depends on the version of Ubuntu you are running.

• Ubuntu 11.04 or earlier: Use GTK+ 2 themes.
• Ubuntu 11.10 or later: Use GTK+ 3 themes.

If you are running Ubuntu 11.10 or later, you can use the same themes you would use with GNOME 3 (or, more accurately, GNOME Shell). This is because Unity itself is based on GNOME 3 in those versions. Unity in Ubuntu 11.04 and earlier is based on GNOME 2, so it uses a different set of themes.

Icon sets should be completely compatible with any Ubuntu release under any GNOME-based desktop environment (including Unity). Also, there should be no theme differences between Unity 3D and Unity 2D.

You can get themes and icon sets from sites like gnome-look.org or deviantART. After you download your desired theme sets, see How do I change to a theme not listed in the Appearance screen? or How do I get and install more themes, icons, and pointers? for information on how to apply them.