The main theme could be the exploration of nature and the human connection to it within the context of national parks.
The main theme of 'Where the Fire Falls: A Vintage National Parks Novel' is likely a combination of adventure, self-discovery, and the appreciation of nature's wonders in the backdrop of national parks.
I think the main theme might center around the characters' personal growth and discovery while experiencing the beauty and challenges of the national parks. It could also touch on conservation and the importance of preserving these natural spaces.