I am constantly customizing my desktop with different themes, colors and wallpapers. I'm going to list here my five most favourite Gtk themes.

Shiki-Colours

Shiki-Colours: The orange themeThis is meant to be a hybrid theme between a dark theme and a light theme. So it isn't too dark, or too light. I find dark themes to be a bit depressing, so this is a very welcome theme to my collection.

It is available in various different colours: orange, blue, green and red. You will need Murrine SVN for it to work, but a Clearlooks based theme is also available. This will work with any distro using GNOME. If you're using Ubuntu Jaunty Jackalope (9.04) you're fine. More information is available on it's page on Gnome look.

Personally, I think that orange theme would be great on Ubuntu

If you don't like dark themes, you can try out the new "GNOME look-a-like" theme, which resembles the GNOME default theme: Clearlooks. It is very compact, and has high contrast. It will be better for you if you have a low resolution or dislike dark themes. I won't be putting it on just yet. I still need to make a few adjustments to make it look a bit better