Currently since there is only one theme you can only change the color scheme for the theme but in the future when more themes are available for XP you will be able to change which theme is used in the same place in the windows registry.
Start regedit, if you are unfamiliar with regedit please see our FAQ.
Navigate to HKEY_USERS\.DEFAULT\Software\Microsoft\Windows\CurrentVersion\ThemeManager
Locate the key ColorName
Right Click on it and select modify.
Change where it says "NormalColor" to "Metallic"
Click Ok, and exit regedit
Note: To go back to the old color scheme durring startup just change "Metallic" back to "NormalColor" Once more themes are released you will be able to modify the DllName key to select different themes.