The theme might be about perception. Her green eyes could cause others to perceive her in a certain way, and the story could explore how she deals with those perceptions. For example, people might assume she is exotic or mysterious because of her eye color, and she has to navigate those assumptions.
Another potential theme could be transformation. Maybe the girl starts off as a shy or ordinary character, but as the story progresses, her green eyes become a catalyst for some sort of transformation. It could be a physical, emotional, or spiritual transformation. For instance, her green eyes might lead her to discover a hidden talent or power within herself, and the story could be about how she uses this new - found aspect of herself to change her life or the lives of those around her.