One of the themes in the book is fighting against wrong doing. Cal does this through the whole book. He saves a rabbit from being tortured. He convinces his dad to call the cops on the Mr. McClusky for animal cruelty, and finally he finds out that Gretchen's father has had a baby with her sister. So he tries to plant evidence of alcohol in his car and drive it into a store. The father later gets arrested for it and also stabbing a policeman. The mother then testifies that her daughter had a baby with the father and the father killed it. The father is then sentenced to have life in Jail. All of this is because Cal fought against the wrongdoing of people. He sacrificed his popularity to help someone in need. This relates to life now with bullying. If you go out of your way to help someone other than yourself you can make a huge difference in someones life.
Another theme would be standing up for the "stupid and crazy." This also relates to the book and bullying. People might think that what you are doing is pointless and that you might lose your popularity over it, but that should not stop you from doing the right thing. Cal risked everything to help Gretchen have a safe life even if it meant risking his. If a lot of people did that the whole world would be a better place.
Another theme would be standing up for the "stupid and crazy." This also relates to the book and bullying. People might think that what you are doing is pointless and that you might lose your popularity over it, but that should not stop you from doing the right thing. Cal risked everything to help Gretchen have a safe life even if it meant risking his. If a lot of people did that the whole world would be a better place.