1. Take a backup of the framework database.

This could be named Passageways, Production_Portal_PortalFx, Sandbox_Portal_PortalFx depending on when your portal was installed.

2. Run the script below on the framework database to see current theme assignment. (Optional Step)

/* Query to see what user themes are already assigned

select * from pwuserpreferences
where AppID = 'passageways.portal' and Preferencekey = 'usertheme'

3. Replace the query from above with the script below to delete current theme assignments.

/* query to delete existing theme preferences

delete from pwuserpreferences
where AppID = 'passageways.portal' and Preferencekey = 'usertheme'

4. Replace the delete script with the script below to assign the new theme to all users. 
/* query to insert Swoogle theme preference for all users; Make sure to change 'Swoogle' to the correct theme name... Case senstive - watch out!

Insert into pwuserpreferences (UserID, AppID,PreferenceKey, PreferenceValue)
select userid, 'passageways.portal','usertheme','OnSemble' from pwuser