These are words which crop up in cryptics with some regularity, but do they both mean the same thing? Or is there already an established difference between the two?
I ask the second question because I can’t say I’ve ever seen such a difference stated. And if it doesn’t presently exist, maybe there is an opportunity to bring it into being; after all, the words are similar and it would be easy to use one while meaning the other. For now I can only give you my own take on it:
A themed puzzle has a number of answers (or even an aspect of the clues) with something in common. It might be capital cities, songs by a particular band, or not subject-based at all – how about a set of clues which all start with the same word? The point is, though, I think of a themed puzzle as one in which knowledge of the theme isn’t a requirement of solving. Often it’s just a bonus feature for the solver to discover and enjoy.
In a thematic puzzle the solver has to work out the theme, failure to do so making the puzzle almost certainly beyond solving without inspired guesswork. Such demands are usually restricted to Listener-type barred puzzles, but not always. You sometimes see daily cryptics with a short preamble which states that a number of answers share a theme and that their clues contain no definition. I’d call this type of puzzle thematic.
Is there, somewhere, an existing statement of the difference between themed and thematic? If not, perhaps we could make that statement now?
Anaxcrosswords 
I have been riding the same hobby-horse on 15sq for some time.
I do not really recognise your first case as themed at all.Randomly dotting odd words around in both clues and solutions doesn’t strike me as a theme.My bigger objection is that it does not in anyway aid the solving.
Your second category can be very exciting and in the hands of someone like (is there?) Araucaria can be superb. He, probably naturally, doesn’t produce them often any more but a few years ago I can remember being so impressed with the skill he showed and so delighted at the pleasure they provided.
Hi RCW – many thanks for your post.
Your first comment helps to highlight a possible need for accurately defining a difference between themed and thematic. I have to disagree that a number of thematically connected answers (or even clues) doesn’t constitute a theme – if the setter has said “Right, I’m going to base a lot of the answers on the theme of…” then it is a theme, albeit partial. I also can’t concede that it doesn’t in any way aid the solving process. The simple fact is it can – not does, but can. Put it this way; if I set a puzzle where all of the Across answers are e.g. television programmes whose titles all happen to be everyday words and are defined in the clues as such, can we honestly say that it isn’t a theme and that, if the solver picks up on the thread after solving the first 4 or 5 clues, he/she wouldn’t be looking for other TV programme titles while solving the rest?
Some may miss the theme entirely of course. Comments on 15sq suggest you always get a split of those who see a blind theme and those who don’t. My point is that such a theme can aid solvers.
Reading between the lines (just a little!) you also seem to suggest that such blind themes should be done away with. I really can’t go for that.
If a newspaper were to truly live up to its name it would contain news and nothing else; no comment, no letters, no horoscopes (hurrah!)… and no crossword, of course. If a crossword truly lived up to its name every puzzle would be a collection of clues and answers following the same format every day, and solvers would quickly tire of it. Themed and fully thematic puzzles are just additional varieties which are used to add, well, variety. And from a setter’s point of view they add variety to the setting process. There have been accusations in the past that themed puzzles are the setters’ way of showing off, but this is a long way from the truth. The task of filling a grid and clue-writing is a technical challenge, but I know from my own experience that sticking to a single format can make you feel a bit jaded, even mechanical. We see themes as a bit of bolt-on fun but, more than that, it’s a chance to stretch ourselves a little, to move beyond (that awful phrase) our comfort zone. We’re not saying to solvers “Look what we can do”; rather we’re asking ourselves “Can we do this?”.