In my opinion the 4 themes of the book are Geographic success and failure, the development or progress of civilizations, Race vs the Environment, and conquest. I am having trouble with this because I don't know if these are the right themes. I also don't know what the overall implications are of the themes for world history are.
I agree with #2 mainly. I think the biggest implication of this brilliant book is the way that again and again Diamond explains the superiority of various groups over others through geography and nature, completely debunking the myth that superiority is based on genetics and culture and that one culture can be thought "more intelligent" than another culture. This message is still just as important and valuable to hear in today's world, even though time has moved on significantly.
I can't argue much with those themes, though I am not sure that race v. environment is really any different than geographic success or failure. I think that you should be sure, though, to get the importance of agriculture in there as a theme. I think that maybe you could replace race v. environment with that or maybe conquest. Anyway, Diamond certainly does spend a lot of time talking about why it was so important to have agriculture.
Implications? That's hard because they're largely in the past. But one implication is that we should not pay too much attention to things like culture when we are trying to figure out which societies will succeed. We should also not think that some races or cultures are superior to others.