Both share the theme of enslavement versus free will. In both societies, the common citizens are reduced to whatever the ruling elite wants them to be, while the elite continue to retain their own free will. The difference in theme are that in 1984 the elite emphasizes the labor of the common citizen, counting on their lack of intelligence, while in Brave New World the elite sacrifice creativity, spontaneity, individuality and imagination for a controlled society, though they court the intellect of the Alphas by showing them the margins of "primitive" cultures.
I would say that the main theme that these two books have in common is that people cannot create a perfect world. In both books, you have governments that have tried to create utopia in some sense. They have tried to plan their society to be just the way they want it. In both cases, they have ended up creating something horrible.
In each book, people have learned to survive by basically checking out. They are not interested in confronting their society’s problems. Instead, they take Soma or keep their head down and swallow the government’s mumbo jumbo. No one faces reality.