Hello Erco, and welcome to Seven Forums.
I'll create a separate tutorial on this tonight with more details. For now, you can do it through Group Policy
at the location below. The saved .theme
files of a user are located in the hidden C:\Users\User-Name\AppData\Local\Microsoft\Windows\Themes
folder, and these: Themes - Regional
Hope this helps for now,
UPDATE for New Tutorial:
Theme - Specify Default Theme to Load for New Users