Thanks for the tips, folks. That was a real now-yo-don't-see-it, now-you-do moment. The theme is something I've heard of but am not famiiar with (it's on an increasingly long to-do bucket list), and I don't know that I would have spotted it without some nudges.
However, as with any good themed crossword, knowledge of the theme is not an essential for solving the puzzle, which I found quite satisfying.