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?