The Question and Answer section for Mahabharata is a great resource to ask questions, find answers, and discuss the novel.
Arjuna didn't want to be a warrior..... to act out his dharma, to fight and kill. After Krsna explains that Arjuna's dharma is to be a warrior, he agrees to fight.
Krsna helped Arjuna realize the importance of his dharma.
This is actually a complex question. There are actually hundreds of themes. Mahabharata is a powerful text which provides an insight into traditional Hindu culture. I might consider dharma, cosmic order, to be a central theme.