How is Newspeak one of the themes in 1984? What message was Orwell trying to give by using this in the novel?
Newspeak is the language of the New Order. It is a language that is based on simplifying and eliminating words to make all ideas more clear cut, more black and white. Doing this eliminates "perspective" and makes things either black or white, good or bad. It also can make bad things seem good, like a traditional euphemism.
Orwell makes Newspeak a theme first by devoting so much time to it, and creating such a complete language. He obviously thought very carefully about the importance of this aspect of his novel.
The way in which it is a theme is tied into the "Thought Police". Orwell was suggesting that propaganda was a destructive force in society. People are suggestible to language and to "conforming" to traditional customs and thoughts rather than evaluating the merit of the thought. This theme is carried through Animal Farm as well. Slogans and word choice can change a person's belief structure. A modern example is the use of the phrase "weapons of mass destruction".