Many of the themes found in Frankenstein are timeless, though some, especially the portrayal of the mad genius, are specifically Romantic tropes that may seem dated today.
One of the most relevant themes would be the issue of forbidden knowledge, or even "playing God." Dr. Frankenstein creates life, but in the end, his creation actually destroys life as well. Shelley lived during the Industrial Revolution, a time of major social and economic change and technological innovation. It was not at all clear to thinking people that these changes would necessarily be for the better. Frankenstein's monster also symbolizes science run amok, a fundamentally amoral force that can have bad consequences.
In our own time, science and technology have improved most of our ways of life, but they have done so at a cost. Does the damage we have done to nature and the environment outweigh these improvements? Or should we be confident that science will find solutions to these problems? Shelley thinks that we should look before we leap, and that progress often comes with unacceptable costs. The monster, uncontrollable and violent, symbolizes these costs.