ThemeSync

The ThemeSync extension is a Rotating Theme type application, allowing you to exchange your Pocket PC Today theme (background picture) at scheduled points in time.

Because it is an ActiveSync extension, only around 11 Kb needs to be installed on the device plus the theme currently viewed.
Only 1 theme is installed at any time, but every time you insert the device into the cradle it checks if a make-over is scheduled and downloads a new theme to the device.
The extension looks for themes in a folder you choose on your Desktop PC. It supports both TSK and ZIP files so you can download themes directly from the internet to the source folder.
You can configure the extension to replace the theme at regular intervals, but most likely you will want to set it up to synchronize the files on a daily or weekly basis.