If the files are unpacked, simply copying all the files in a folder on Windows, but before this, do not forget: Disable Touchflo 3d. 
  Start-> Settings-> Today-> Items-> remove the checkmark from Touchflo 3d. 
  Copy the theme file in the Windows folder with the replacement 
  Turn touch flo 3d. 
  Start-> Settings-> Today-> Items-> check with Touchflo 3d.

  If theme packed in a single file with a theme, you need to use  Diamond TF3D Config
With the software Diamond TF3DConfig  you can change the order of tabs in the TouchFLO 3D, the display format of the date, language, change the theme etc ... Your HTC Touch Diamond.

-How to install the Theme:

- Create the root of internal storage of your HTC Touch Diamond a folder named: Themes

- Unzip the downloaded file and save the file. Theme in the previously established .

-Set the image format. Png for example on the internal storage, My Documents, My Pictures: Selecting the tab: Settings, Wallpaper of TouchFLO 3D.

-Open Diamond TF3D Config, go to Appearance, Select Theme, click Path, double-click Storage, click the Themes folder, do OK. Choose the theme in the menu that appears under Path. Click Apply and then click Start. Wait for the installation of the theme, it takes several minutes. Click Finish then click OK to restart your Pocket PC.

- To retrieve the original TouchFLO 3D, use the "Restore defaults" Diamond TF3D Config.

About Diamond TF3D Config 0.6.7.

Download.