Proms are a rite of passage for many high school students. Some of the popular prom themes are Midnight in Paris, Midsummer Night, Underwater Paradise, and there are many more themes in which high school student’s dance to every spring. Prom committees only pick one theme. Could you imagine a western-under-the-sea prom? Nope, it’s not going to happen. (But on second thought, I wonder if I could write a western-under-the-sea prom story?)
In the Wikipedia page for science fiction, there’re at least one hundred themes listed for the genres. Some of them are steampunk, cyber warfare, space warfare, and alien invasion. How many other themes are there for the other fiction categories? Just as many. Are the mystery themes used by Agatha Christie and James Ellroy different? Of course they are. Hercule Poirot does not belong in a James Ellroy neo-noir type mystery novel James Ellory write.
As a flash fiction writer you only get one theme, make it the right choice.
You may want to look at the themes Wikipedia list for science fiction:
For the fun of it: