Hi Tim, great minds think alike!! I had this very same thought last night.
I am currently going through our collection, redoing photos and adding new arrivals. For a change, I decided to work backwards and have so far got to the 1999 smurfs. Well, last night I was trying to work out exactly what you thought - when did the themes start?
I think you are right Tim, the themes did seem to start with the Marching Band smurfs in 2002. 2003 saw the New Generation and although there is the exceptions of the new basketball smurf and the hippie, the others all seem to be very modern looking. Then 2004, 2005 and 2006 all have very obvious themes.
Before 2002, I can't really see a theme for a year which included all the new releases although there are quite a few pairs that have been released eg. snowboarders, disco, horse riding, caveman, bridal etc.
I wonder if the themes are going to continue? I suppose in many ways, it is easier to work with a theme when trying to decide what should be made. It will be interesting to see.
What about the historicals?
The Historicals are a good example of an older theme but I would say these are an additional set as there were also quite a few other different smurfs released the same year.