Though Mary Shelley wrote Frankenstein nearly two hundred years ago, many of the themes of this novel are relevant to today's society. Let's consider the themes of nature and acceptance in society.
To begin, the theme of nature is characteristic of Romantic literature. The natural world offers both beautiful and harsh environments, endless variations which inspire awe. Romantic writers like Shelley emphasized the value of the natural world as a response to the effects of industrialization on society, which held connotations of forced conformity. To revel in the beauty of nature, even if only through the written word, was to "get back in touch" with the world and our own humanity.
This brings me to my next point—not all of the interactions between mankind and nature were, or are, pure and revitalizing in the way some Romantic writers portrayed them to be. Shelley confronts the theme of man versus nature through Victor Frankenstein's attempt to bring natural processes into his own hands. Frankenstein's experiments in re-animating cadavers and the creation of his Creature only temporarily gives him the control he is seeking. He is afraid of the abomination he has made by violating natural laws of life and death. Ultimately, Frankenstein realizes he must set things right by killing the Creature and he begins to hunt him down. Before this can happen, he succumbs to the harsh environment of the Arctic. Rather than being refreshed by the experience of nature, Frankenstein attempted to take power over it, and in the end was subdued by the powers he failed to control.
Nature is an important theme in our lives today, despite the fact that much of the world's population live in industrialized settings. Someone who has grown up in the city can still feel the joy and revitalizing quality of a trip to the mountains, sea, or countryside. One of the hallmarks of modern, working life—the vacation—is intended to provide exactly this kind of soul-refreshing break people desire! At the same time that we enjoy nature, many people fear it, too. Our environment is suffering rapid degradation as a result of industrialization and it is becoming more and more difficult to ignore the very real possibility that we humans may be responsible for our own undoing. Similar to how Victor's own creation steals away his happiness and indirectly causes his death, human activity may create an environment where we have erased our own possibility for a joyful existence. We, too, may succumb to nature.
The second theme I would like to discuss is that of acceptance in society. When Victor Frankenstein begins to share his story, we understand him to have been a man possessed by his work. His interest in the sciences always made him a little on the outside, and during his work on the Creature, he was essentially isolated. Frankenstein's work repeatedly forces him to the fringes of society—first in his dedication to his experiments, then after his repulsion at the Creature and resulting illness, and again when he realizes the Creature has committed a murder. Deep down, Frankenstein knew that his experiments went against the moral code of his society. If he had been working at something else—say, a cure for tuberculosis—his fervor and dedication might have been more acceptable. Even if he was reanimating corpses for some other reason perceived as valuable to society, rather than his self-serving interest, it might have been permitted. Frankenstein can never really come clean and be accepted in society because his work was such a violation of social norms.
The Creature is a much more tangible example of acceptance or failure to fit into society. For one, he is singular in having been made from the corpses of others, and is really alone in this unique quality. There is no one who can properly answer his questions about the world and what it means to be alive because he isn't even really "human." Above all, the Creature suffers because he wants affection and companionship, but there are no other beings like him who can be his equal. All of that aside, the Creature's physical appearance gives him away as something strange and repulsive. While his internal emotions make him feel separate from society, his appearance makes society want to separate from him.
Frankenstein has been referenced many times in the field of disability studies and in discussions of what it means to be an abnormal or "monstrous" body in relation to society. Many social scientists argue that we conjure up ideas about monsters in terms of self-definition—we are this, not like that. In this way, the Creature reinforces ideas about the normative, whole, and substantial body which conforms to notions of appropriate development and restraint from excess. In the present day, this sort of "us versus them" mentality persists and has resulted in entire sects of society where people are simultaneously accepted and dehumanized.
If we think of the Creature as an archetype for the deviant body—see Foucault's idea of the "docile body"—we can draw many parallels between the novel and the present-day treatment of people who are disabled, ill, or otherwise not "docile." For example, many neurodivergent people are actively trained or encouraged to act, think, and feel in a way which is contrary to their reality. The Creature learns to make himself more acceptable to others through speech and reading, but his physical appearance remained an obstacle. People run from him in terror—he is only safe when he is alone. Similarly, people who live with mental illness or autism spectrum disorders are often able to learn ways to "blend in" and become more acceptable in society, but are punished when they fail to do so. Throughout history, isolation has been quite literally forced upon neurodivergent people through institutionalization. Today, forced hospitalization is a very rare and extreme punishment for failure to conform to societal norms, but many people experience regular conflict and dehumanization when their condition does not blend seamlessly into social interaction. As I described above for the Creature, personal feelings of guilt or shame surrounding neurodivergence can make a person feel separate from society, while visibly divergent thoughts, feelings, and actions may make others desire to be separate from the individual.
These are just two of the major themes addressed in Mary Shelley's work, but I hope that it gives you an idea of why this novel remains so powerful in the present day.