One possible theme is hope. Just as the firefly's light pierces through the dark, it can represent hope in a difficult situation.
The theme of resilience could also be present. A firefly has to keep shining in the dark, regardless of the vastness of the darkness around it. In a story, this could translate to a character who endures hardships and still manages to show their strength and positive qualities. It could also be about the beauty in small things. A firefly is a small creature, but its light in the dark can be extremely beautiful, and the story might focus on how we often overlook small but beautiful aspects of life until they stand out in a moment of 'darkness'.