The main theme in Louis Sachar's Holes is the theme of Justice. Justice is what Stanley wants and expects, and the way it plays out is what makes the story work. The book says that justice will be done, although it may take a very long time and may not happen in the usual ways.
Another major theme is friendship. The friendships that Louis discovers in unlikely places help him understand the world and himself.
The main theme of the book is trust. Stanley and Zero trusted each other thru the whole experience. Soon the trust turned into friendship and Stanley decided to help Zero find his mom.
As mentioned by the great answers above, the main themes in Holes revolve around justice and friendship.
I will start with Justice. In the beginning, we get insight into Stanley Yelnats' crime of stealing shoes. Although he is sent away to the camp for juveniles because of this act of crime, readers know that he is actually innocent, and that this act was actually not at his fault. When the Warden is caught practicing illegal ways at her camp, Stanley has not only been freed of the camp, but justice has been served. Not only is he once again a free man, as he should have been the whole time, but the Warden also gets justice served to her as well: as she is now going to be locked up after the type of struggle she put the boys through.
Friendship is also a key theme in this novel. Stanley and Zero form an instant friendship when they meet. Both are a bit outcasted and not as accepted from the rest of the boys at the camp, so their friendship is really what carried the two throughout their journey. It is a strong bond that these two form that really make the book impressive and enjoyable to read. Readers are able to relate to such a bond, and can enjoy reading as they find success together.
More information on these themes can be found on eNotes' Holes webpage. I'll include it in the link down low!
friendship, justice, how the past and present effect eachother and how good wins the battle against evil...