Using relative word frequencies to make older sentences sound slightly more modern was interesting, but there’s no reason the process has to run only one way. We can use the same technique to make modern sentences sound slightly more gothic by looking for synonyms that better match the word frequencies found in Frankenstein.
Original: It was dark and spooky inside the abandoned house
Processed: It was dark and spooky inside the deserted house
Original: The professor was spending too much time in his lab working on dangerous experiments
Processed: The professor was passing too much time in his laboratory working on grave experiments
Original: The girl was angry when she saw the ugly dog being mistreated
Processed: The girl was wild when she saw the wretched dog being mistreated
That certainly sounds a little more Gothic, but not by much. Perhaps things would be more interesting if we instead look for the rarest synonym actually used in Frankenstein rather than the most common. After all, one could argue that the unique words an author chooses are more important to their style than the words they use over and over again.
Original: It was dark and spooky inside the abandoned house
Processed: It was sinister and spooky inside the empty building
Original: The professor was spending too much time in his lab working on dangerous experiments
Processed: The professor was spending excessively much time in his laboratory operating on serious experiments
Original: The girl was angry when she saw the ugly dog being mistreated
Processed: The miss was enraged when she checked [that] the vile dog [was] being mistreated
A little bit more interesting, but once again demonstrates the limits of trying to capture an author’s style word by word rather than phrase by phrase.