One theme of Achebe's Things Fall Apart is the importance of customs and traditions. Okonkwo, through much of the novel, is attempting to balance Igbo traditions of leadership and masculinity with the new influences that are making their way into his culture. The reader is shown a great many traditions and customs during this book. For example, the use and meaning of sharing kola nut between men is a favorite passage of mine about tradition and custom.
Another theme is masculinity. The novel does a nice job of presenting the faults of hypermasculinity. For example, Okonkwo wants to be a better man than his father, who Okonkwo saw as weak. In order for Okonkwo to not be weak and show weakness, he believes that he must be aggressive and violent to a fault. He beats his wives and even volunteers to violently kill his surrogate son in order to not appear weak. Okonkwo looks down on men that he considers have effeminate characteristics because he sees it as weakness.