As lumen explains, the key is the specific crossword meaning of ‘thematic’ - the grid will be peppered with references in answers and sometimes clues to the theme, often cryptically hinted at by the title, meaning the clashing cells are of a kind (days of Christmas, knights of the Round Table, celestial bodies or whatever).
So your tale might be joined by corset and tonk, which don’t make a theme, until you realise - the ‘pdm’ - that the title Love all means that it’s all about ducks, obviously, and all your other clashes suddenly fall into place.