Choosing a main theme is difficult because it is a subjective choice. Different readers are going to see different themes as central to the story. For example, a major theme that I see now (but didn't as a child) is the theme of materialism. Toad is a wealthy, playboy-type character, and having money, spending money, and owning nice things is how he pursues happiness. Granted, the book shows Toad's tendencies as not something that is positive about him, but the theme is present throughout most sequences that Toad features in. I think other central themes are the themes of friendship and forgiveness. I have a hard time separating these two themes from each other because I believe that they are very tightly woven together. Characters are friends with each other, and they go out and have adventures with each other. Additionally, they love and support each other, and a big part of that is forgiveness. None of the characters are flawless characters, and they make mistakes both big and small; however, the characters are always willing to forgive the faults and mistakes of another character even when that character might not deserve it.

There are many themes in this book.  The eNotes summary and study guide for the book has an excellent discussion of the themes.  Among the themes that it lists are:

• Hospitality.  You could also call this generosity.  For example, we see that Rat lets Mole come and live with him even though Rat has only met Mole that same day.  He allows Mole to stay with him a long time.
• Forgiveness.  Characters are forgiven for bad things they do, whether big or small.  Even the weasels are forgiven when they show that they feel sorry for what they have done.
• Compassion.  Characters tend to understand the others' needs and act to try to help others.  For example, Badger, Mole and Rat try to help Toad get over his mania for driving.