A easy way to insert a theme to your unattnded is:
Lets say you want the Royale theme:
1. Install the theme on a refference PC (and customize it if you like)
2. Then Copy the theme file from C:\WINDOWS\Resources\Themes (for royale this will be the "Royale.Theme file") to your \$OEM$\$$\Resources folder.
3. Then Copy the theme folder (for royale this will be "C:\WINDOWS\Resources\Themes\Royale" to to your \$OEM$\$$\Resources folder
4. Then add the following lines to the "WINNT.SIF" file: