Right from the link that you posted:
The screenshots on this page are contributed by various Gentoo users. Information about the themes used, background images, etc. is not available through Gentoo. You will need to contact the author of the screenshot (if you can find this person) for more information.
You need to either find the information yourself, or contact the author.
That said, if you want Ubuntu, there is little use in looking at Gentoo's screenshot contest. The culture is so different: Ubuntu is supposed to be easy, while Gentoo aims to be customizable (and not easy). You'd better look at Ubuntu themes (maybe this and this or something else), or be prepared to craft your own theme.