Stefano and Girish inform us that the official Windows Media Center theme (called Royale) can be used on Windows XP. Cool!
I like tweaking my colors but am not willing to load themes from 3rd-party sites on my work PC (or don't want to get busted), so it's nice to have an option. There's instructions on where to copy the theme to in the RAR file, which can be downloaded from Winbeta.
UPDATE: Stefano alerts us that the theme has been officially released (called “Energy Blue“), and can be got here. You'll have to extract the relevant files with WinRAR and copy them manually to your Resources directory, though, as the installer is designed for Tablet PC's only.