The overarching theme of Frankenstein is the dangers of playing God.
To be clear, this isn’t about the dangers of science or knowledge—it’s about the dangers of trying to take on a role that you’re not truly ready for. In the case of Shelley’s novel, it’s Victor Frankenstein’s quest to play God by bringing life back to the dead. He doesn’t understand how complex life is, and he doesn’t realize how much he doesn’t know about his own limits as a human being. He makes mistakes along the way, and those mistakes have real consequences both for himself and for others.
This theme also applies to characters like Elizabeth Lavenza and Victor’s father, who try to make their own decisions without considering how they might affect others around them—and end up paying dearly for their lack of foresight.
The novel addresses other themes as well: family, obsession, companionship, morality… but none more directly than this one overarching idea: that playing God has consequences that can be dire indeed if we don’t consider them carefully before we act on our desires!