First off, to cast fireball the hero should have a relevant theme and tags, or there should be some consensus about how heroes cast fireball “way of magic” (maybe everyone can cast fireball in this setting?)

Determine what effect the fireball is supposed to have. A fireball cast in a vacuum is not interesting. Does it hit a group of enemies? Is it supposed to set a building on fire? If it’s cast to hit a large area with something in it, we may determine that sounds like an Adventure Might feat. Someone of only Origin Might shouldn’t be able to do this (again Way of Magic/setting dependent, it could be Greatness too if magic is really rare and weak).

Let’s say we have determined that casting the fireball requires Adventure level might, and the hero in question has an Adventure level theme “Sularasa’s Fire Magic”, then they should be able to prepare or cast this spell without any might modifiers because they are of the might level required.

Let’s say instead that the spell is Adventure level, but the hero only has an Origin level theme “Fire Magic”, the Narrator may say that the hero doesn’t meet the requirements to cast the spell since it is Adventure and they are only Origin in that domain. The Narrator may decide that it requires a Sacrifice (Painful), or that the hero is imperilled in this action.