Having completed the grid, identified the theme and resolved all the clashes, there's something which bothers me.
The preamble says that the theme "explains the rationale for each of the clash resolutions and letter changes". I can see how it applies to every clash resolution and the first letter change. The second letter change does not fit the pattern, but could be explained by a phrase which is closely related to the theme.
It seems odd to me that this is not mentioned at all in the preamble, as it's the only exception to the general rule; beyond being thematically related, I can't see a justification for it. I'm wondering if anybody else is bothered by it, or can justify it -- or wants to tell me there's another place to find the second word which fits the theme exactly!
(I'm entering the "smaller" of the two possible themes as it's the one that applies 18 times in the grid, not once.)