@PaulR said:
Not necessarily. And just to go against the grain in a devils advocate way - why are themes that necessary when writing to moving pictures? I like themes but they're not a pre-requisite of movie scores are they? Themes can be quite hidden in their structure - look at Thomas Newman for example. He writes themes all the time but most people don't remember them as a bona fide theme and walk down the street whistling them. Themes are in there though. It's perception.
Besides, what good is a theme at a basic level? Take the thematic material of The Magnificent Seven. The theme (according to Bernstein) is quite a simple thing and like simple things is quite memorable. But that theme would be nothing if it wasn't for the tremendous orchestration/arrangement that's going on around it. And there are many themes in that film. So the question you could have asked would be more pertinent to orchestration/arrangements rather that simply just thematic material. Anyone can write a theme or a tune (not necessarily memorable) but doing the stuff around it is the really difficult part.
This is, I believe, of general importance: there is no guarantee for anything in writing themes per se, nor are they a prerequisite for composing music of substance (that is, of beauty). I suppose nobody taking part in this discussion has to be reminded that some of the most refined music known to us is completely or largely athematic - Beethoven, Anton Webern or Morton Feldman coming immediately to my mind as being particulary strong cases in point.