The most important theme in The Hunger Games is freedom versus oppression. The entire point of the Games is to keep the citizenry under control, demonstrating that they have no freedom to do as they wish. Forcing their children to kill each other on public television shows the power of the Capitol, and tells the citizenry: "If you revolt, we will crush you."
Electricity in District 12 comes and goes, usually we only have it a few hours a day... The only time you can count on it is when they're airing the Games or some important government message on television that it's mandatory to watch.
(Collins, The Hunger Games, Google Books)
By limiting resources and starving their populace, the Capitol is able to keep comfort and abundant resources for themselves, and not worry about violent uprisings. Hungry people need to focus on finding food and staying alive, and cannot concentrate on higher issues like government or freedom. When the people of the Districts find a symbol to follow -- Katniss -- they are able to fully contextualize the extreme oppression of the Capitol, and find the power to rise up against it.
One of the most important themes is perserverance. Katniss must learn how to make it through tough times and to find strength not only within herself, but inside of others.
If you are looking from a perspective of romance, especially if you've read the entire series, you could state that the theme of the series is that love conquers all. No matter what Katniss and Peeta had to go through, their love for each other helped them find the strength to keep going.
Struggle for safe existence in the midst of unhealthy competition seems to the main theme in Hunger Games. It is reinforced by diverse themes. Tyranny counterbalanced with sacrifices is one of these themes.
Othes sub themes of Hunger Games are quite challenging and they focus the pivotal point. Evils and the repressive government are depicted as monsters. The writer in Hunger Games reveals the dystopic vision of the world. Children are shown in violent circumstances. Their psychological trauma results from this. The dogs symbolize monstrous tendencies. The struggle of survival and safety is also very much important.