There are several possible themes, but Kurt Vonnegut tends to focus on the possibility of equality being enforced by the government, to the detriment of everyone. So, in one word, equality is the theme, but in more detail, important questions about whether sacrificing happiness, independence, freedom and personal achievement at the cost of "equality" is worth it. Vonnegut definitely asserts that it is not, and implies that forcing everyone to be the same is, first of all, impossible, and in addition, dangerous, unhealthy and inhumane. No matter what measures you take to force people to be the same, people will always be unhappy, just in a different way.
This theme of equality can be seen in small ways in our society today, where being nice and giving warm fuzzies are often given more importance than encouraging achievement and individual success. We can learn valuable lessons from Vonnegut's dystopian predictions that can be applied in our own lives today. I hope that those thoughts helped; good luck!