I know how to write one when given a specific theme but when you do a compare and contrast essay your using multiple themes of whats similar and whats different.
You need a subject and an attitude toward that subject. I would suggest you start with the face that the 1931 Karlof movie bears very little resemblance to the Shelley novel, the 1996 DeNiro presentation follows the novel much more closely until the end with the "resurrection" of Elizabeth.
So your thesis sentence might look something like this:
"Whereas the 1996 production of "Frankenstein" remains faithful to much of the action and many of the essential themes of Shelley's novel, the 1931 production uses only the barest elements of the original plot and does not seem to even attempt to follow Shelley's original theme."
You might also focus on the fact that until the DeNiro version, no movie accurately portrays the creature as a living, thinking, feeling creation. All we get is the monsters who moan and walk around as if they have no knees or elbows moaning unintelligible syllables. I always wondered why they went that route when the creature in the Shelley's book is obviously an intelligent being, capable of learning and reasoning. Does it just make for better film? I don't think so. The creature as portrayed in the book and the DeNiro version makes for a much more intriguing character--one we can relate to and sympathize with. That might be your thesis.
"Movie versions of Frankenstein fail to adequately portray the complexity of Mary Shelley's creature until the 1996 production where the creature is portrayed by Robert DeNiro."