The theme is not yet in the normal extras repository, which is enabled by default on your N900. Instead, the theme is still in the extras-testing repository.

The extras-testing repository can be enabled from within the Application Manager, but you should know that applications in extras-testing can have bugs that can make your N900 misbehave.

There are good explanations on these things and more here: http://wiki.maemo.org/Repository

Here's a page specifically about extras-testing: http://wiki.maemo.org/Extras-testing