What is a theme for 1984?
The enotes link below features several examples of good themes present in Orwell's work. I think that one of the most compelling depicted is the idea of how modern conception of reality can be steeped in the idea that the drive for perfection leads to a drive towards destruction. Oceania's view for the perfect world, free from all threats, is a domain where the government appropriates the world in accordance to its own subjectivity. In this process, individuals are secondary to the idyll of success, which is stressed more than anything else. A step towards its creation is a step towards the destruction of individual freedom and autonomy.
To identify your theme, start with this question: "What is a 'big idea' in this text that would be of interest to any person, any where, at anytime?" Based on that question, some clear themes in 1984 would include: freedom, government control, police states, education, propaganda.
To fine tune your theme for analysis, construct a phrase about one of the above that illustrates its use in the book. For example:
freedom of thought as an act of revolution in a totalitarian state
propganda as a means of thought control
government control of education
You will have identified a good theme to write when you meet the following criteria:
(1) The big idea of your theme is a subject of interest to any person, any where, at any time; and
(2) Your phrase about the big idea can be supported with multiple examples from the text.