The main theme is that if we are not careful with our language, which is the way we interface with the world of things and the world of thoughts, then we will be at the mercy of almost anyone who uses language against us. This was a major concern for Orwell. The other is that if we do not take the trouble to protect our knowledge of the past, it can be "recreated" to make us believe almost anything. That is the meaning of "Who controls the present controls the past; who controls the past controls the future."
These two things are not separate. Controlling the past is, in part, achieved through controlling the language that we use to describe it. How many of us actually know what has happened during their lifetime, never mind the history of our country. We are told (usually by the opposing party) that everything the other party has (almost) ever done is erroneous, ill advised, or just plain wrong. To use the weapons of mass destruction example, it is my recollection that almost the entire world believed the weapons were there; the way it is told now, Bush made up the whole thing. Who will remember in 20 years what actually happened if it's unclear just 5 years later? This is already being used to criticize Bush (about whom no one will care in 2 years anyway), but is it valid or just a tool?
"Who controls the present, controls the past."
1984 is a cautionary novel which presents the dangers of a totalitarian government. A state which controls the perceptions and thoughts of people destroys what is uniquely human in each person as it destroys the private life of the individual. In the novel, history is edited so people believe there are causes for the unfounded actions of the government. In our society the Bush claim that there were, in fact, weapons of mass destruction in Iraq launched a war costing the U.S. billions of dollars and the lives of many of its citizens. In addition, personal liberties were threatened by the Patriot Act.
Manipulation of popular feelings and ideas can be accomplished by the mass media. In 1984 Big Brother on the "telescreen" tells people what to think. When Winston writes "Down with Big Brother" in his diary, he commits "thought crime." Nowadays people's being attacked for their politically incorrect ideas is equivalent to "thought crime" because people have lost their jobs for using certain words or expressions. In our society, an independent journalistic group reported that the media presented Obama in a more favorable light than they did McCain. "Time" and "Newsweek" both had far more covers with Obama than with McCain, as well. This manipulation of popular feeling and ideas by the mass media is one of the situations about which Orwell warned.
The main theme would be that the control of one's thought or perceptions results in the lost of one's individuality, whether intentionally or unintentionally.