We have historically allowed users to choose from several themes, but now we would like to force all users to use Carbon.

If I limit the COS to only this choice, it is effective in limiting users from changing it in the future, but if the user is already using another theme, it does change the current theme to this choice.

Is there a zmprov command or another method that I could use to switch all to the default COS value?

To reset a key to Cos or to Domain default value, you can set the key to null with a command like this for zimlets but not sure how to do it for themes:

Code:
zmprov ma <user@domain> zimbraZimletAvailableZimlets ""