[horde] Affect all user's preferences

[EB]

I was careless when I first started a thread and it did not have a subject.
Someone else has made the same mistake so we have more then one topic on the
original thread.  So, I'll start a fresh one....

Is it possible to affect all current user's preferences for theme?  I recently
changed the theme in horde/config/prefs.php and it has changed for the login
screen and new users.  However, existing users were not affected.  Is it
possible to force a change on all?  If so, how?

Thanks!