The overriding theme of power seems to dominate Orwell's work. Its premise surrounds how Napoleon and the other pigs run the farm and consolidate their power and control over the other animals. The theme of how power becomes essential for all governments is a critical element in the work. In this sense, one can see the demonstration of the idea that power can come from "the top down." At the same time, I would say that another theme is how this power is used to suppress individuals who might wish to exercise some of it from "the bottom up." Squealer's distortion and control of information and the fact that Napoleon's structure only works when all others in the farm are similar to Boxer, sad creatures who blindly follow the government and give everything, including their lives, to it would represent this theme. Within this theme is the idea that government must remain in control through ensuring a blind following from its people and this must be done at any and all costs to ensure that the focus of power does not leave the government.
Another theme, important to Orwell in this book and 1984, is the malleability of the past and our responsibility to know as much as we can about it. The pigs change the "commandments" whenever they need to to justify their actions. Since the animals have "many" generations in the same time period that we have one, there aren't many original animals left who know what the commandments were, and the others seem willing to believe anything.
I think this theme is particuarly relevant in our world today. In our world, we are faced with a deluge of information from the internet, news programs, and publications. When politicans blame the past for present situations, when the site past events as precedents for current decisions, do we know tht they are correct? telling us the truth? willfullyl/accidentally slanting the facts for their purposes? The same is true for the news media? (I would suggest you read William Safire's "Scandalmongers" for an interesting study of the role of the media during the early days of our country --- you might be surprised how some things never change --- if he's telling us the turth :))
In 1984, the government has an entire department to invent the past to justify the future. I think we need to be careful that we don't have one too.
There are a lot of themes in this book. But if I had to pick one that would be the main theme of the book it would be this: the main theme is an exploration of how totalitarian governments keep control of their people.
There are a couple of ways that we see this in the book. First, we see that they keep control of the people by lying to them. The pigs keep changing the commandments, for example. Also, Squealer tells lies to make the other animals believe that things are good or that Snowball is bad, or whatever else Napoleon wants them to believe.
Second, they keep control through violence. We see this mainly in how Napoleon has the nine dogs that he uses to enforce his commands.