My tanks temps change with the seasons.
Things like warm and cool are all relative. This is the way I see it: there are 4 temp ranges for fish - hot, warm, cool, cold. Over 80 is hot, under 70 is cold, 75-80 is warm, 70-75 is cool. The natural temperature ranges for most fish allow them to comfortably exist in 2 of the 4 categories. Bettas no exception - warm or hot. Cool is okay but not somewhere I would keep a betta long term, though for a season it's proven to me not to be problematic.