If your are not satisfied with the default theme of Ubuntu 10.04 "Lucid Lynx", then community themes from The Artwork team is a few commands away. These are completely out of box kind of themes to make your desktop look beautiful and appealing. Let's go straight to installation.
Open a Terminal window (Applications -> Accessories -> Terminal) and paste these commands on command line:
sudo apt-get install community-themes
sudo apt-get update
Now are done with theme installation. To change the theme, goto ( System -> Preferences -> Appearance and select the Theme tab.) You are right there to choose a new theme for your personal desktop.
Check out the sample theme below..