Rocky7 - all the clues you mention have a thematic element.
16 parses as 3,3, the first half being thematic. Def is last word, solution being an example.
17 parses as 1,1,4.
20 parses as 3,3, the second half being thematic. Def is last 4 words.
Hi again, chrise: yes, I’m afraid that’s not the theme. As you suspected it’s quite a bit broader than that, though it does include a chemical element (haha).
I had the wrong answers for both 21 and 25a. 21 is unforgiveable because the answer I had meaning a specific name in a well known play is not even spelt like that. But it does parse rather well if you can think what that is.
Anyway once those were sorted it all fell into place. Thanks for all hints on here and thanks to Brendan for a cracking prize. Though I'm still not convinced about the theme.
This took me a long time to get through, especially the LOI of 24a (which took up a third of my solving time). While the theme helped in several areas, the grid was unhelpful, there was some very tricky wordplay and some obscure definitions to identify. Seemed to end up with loads of vowels as crossers.