Sorry about the title, couldn't help myself.
If you haven't seen the new branding (samples here at https://wiki.ubuntu.com/Brand) and you aren't running the Ubuntu Lucid Lynx Alpha-3 on trial (http://cdimage.ubuntu.com/releases/lucid/alpha-3/), you can still get new themes for Light - Ambience and Radiance. These are available and working for both Lucid (9.10) and Karmic (9.04).
If you are running the older Ubuntu version such as Karmic, install the latest Murrine GTK+ engine and then install the themes from the first 2 links in the beginning of the post (how-to at http://www.webupd8.org/2010/02/install-proposed-community-themes.html).
Chromium theme for Radiance also available: