It is difficult to say that there is a single universal theme in any author's work, but the closest one could have for Orwell is the relationship between freedom and social organization. This covers "Animal Farm," "1984," "Politics and the English Language" and a number of his nonfiction works. Orwell is continually examining what it means to be free and how that relates to how a society is organized. This extends to things like its economic structure, but also how its language is organized and arguments are made. Think of newspeak, or of the shifting laws in "Animal Farm."