I love the Xubuntu 10.04 theme and I want to install it on my Ubuntu 11.04. It's possible to install only the theme?
Yes. If you want all the xubuntu themes and branding, install the
Be aware that the Xubuntu themes are tested primarily in the XFCE environment: there may be gaps in the coverage when used in a GNOME environment, although it may also be that it works perfectly.
If you are especially attached to the Xubuntu 10.04 theme, and the theme in 11.04 is unsuitable for you, you could potentially forward-port the xubuntu-artwork (and dependency packages) in some private repository (e.g. a PPA). Note that there may be changes in the theming engines or similar that may affect the work of forward-porting these packages.
The specific theme you are looking for is called Albatross it was designed for Xubuntu 9.10 Karmic if you would like a copy of the gtk theme you can download it at the xubuntu-artwork launchpad site here The tar.gz file contains alot of things as well as the them which is located in
hope this helps
As Emmet has already explained it was designed to be used with Xubuntu so there is no guarantee it will work with ubuntu but you can try if it doesn't work you can switch back to another theme