I would set forth some of the main themes of Milan Kundera's The Joke as follows.

First, there is the issue of personal responsibility in opposing an authoritarian regime like that of Czechoslovakia's in the novel. The story opens at a point where the Communist regime is fully entrenched. A message on a postcard that is meant to be ironic gets the protagonist Ludvik expelled from the Party and the university and sent to a labor camp. In the atmosphere of fear and paranoia that prevails, one cannot entirely blame Ludvik's girlfriend Marketa for showing the postcard to the authorities as a means of protecting herself. But the corollary of this theme (i.e., how much we can expect anyone to be personally accountable to a friend) is the theme of fear in a totalitarian regime and its distortion of people's judgments. Ludvik's message was clearly meant to be humorous, but in a dictatorship no one knows any longer what is humorous and what isn't.

A second major theme is that of the rightness or wrongness of retribution. When Ludvik is on the outside again, having served his punishment, he seeks revenge against the leader of the tribunal that convicted him, Zemanek, by seducing Zemanek's wife, Helena. Later, when he realizes this is wrong (it's actually displaced aggression and misogyny that have motivated him), he seems to have learned something about himself and the world, and he seems to have been reconciled to the fact of coping with conditions as they are. The centerpiece of the novel's plot is arguably the Moravian folk festival called the Ride of the Kings. The characters of the novel, Ludvik, his friends Jaroslav and Kostka, and others, have different views of whether the relevance of a traditional celebration such as this can be sustained in the Communist world which has sought to overthrow so much of the past. When Ludvik decides to take part in the festival as a musician, he seems to be attempting to answer this question. But the deeper theme, as in so many works by authors who lived under Communism, is: did the Communist Party have something positive at its root, something that could be reconciled with the past and with ordinary humanity, or was it totally corrupt, existing simply as a means to the absolute power of the Party leaders? Kundera's novel poses this question.