Other than that the themes are related to whatever could be trending that day. Seasonal (spring, start of school, christmas, vacations), important days (Valentine's, International day of chocolate) or something that answers the current issues (BLM, diversity) and sprinkled with regular topics (recipes, home improvements and decorations, beauty, quotes and wallpapers).
You can try to use Pinterest trends (https://trends.pinterest.com/) to see what is trending at what time. There is good chance you'll be able to guess the topics from it.
There are also differences between the tabs in different countries. I've got two of my pins featured before and couldn't find them anywhere. (I've got notification about it through email.) I found out later they were on the Today's tab in India.