There are a number of salient themes in Things Fall Apart. While we might examine the novel's treatment of the basic immorality of colonialism or the nature of friendship, perhaps the most prominent themes of the novel relate to (1) social structure as the basis for morality and (2) identity as defined by purely individualistic, non-social traits. 

Starting with the theme of individualistic identity, we can see this theme at work in Okonwo most clearly but also in his daughter Ezinma. In a social setting where roles are rigidly defined (which we can see in the hierarchy of the wives among other places), identity is nonetheless characterized by personal qualities. Custom plays a substantial role in determining a person's outward position and status, yet individuality often comes to the fore. 

"Tribal custom dictates every aspect of members' lives. The tribe determines a man's worth by the number of titles he holds, the number of wives he acquires, and the number of yams he grows" (eNotes). 

The counter-point to the influence of custom and social codes is found in the concept of the personal "chi."

"Clearly his personal god or chi was not made for great things. A man could not rise beyond the destiny of his chi."

Thus individuality is a foundation of character in the novel, a point which balances out the strict and formalized modes of identity enforced by social custom. This theme is important in part because so much of the novel is dedicated to exploring those social customs. Despite the rigidity and prevalence of these social identifiers, Ezinma and Okonkwo and others are seen, understood and act in accordance with personal qualities that have little to do with social norms and status. 

The theme of social structure as the basis for morality is the more obvious of the themes being discussed here. Okonkwo kills his adopted son and the village sees this as the proper action (the moral thing to do) because the mechanism of village regulations calls for it. Many other examples can be drawn from the text to illuminate this theme as one of the central ideas in the text. 

Importantly, the novel uses this notion of morality to do at least two things. The tribe's moral code is opposed to that of the colonizers and shown to be, arguably, equally arbitrary yet - critically - it is as fully-developed as anything the colonizers might claim. Also, the highly structured moral code creates an animating tension in the novel, which generates much of the complexity of the narrative as characters navigate conflicts of conscience within the framework of social mores. 

One of the major themes of the novel is the conflict between the two traditions, that of the Igbo and that of the incoming Christian missionaries and other Europeans.  The customs of the Igbo that are so horrifying to the Europeans such as the killing of twins or the banishment of certain members of the tribe lead to serious conflicts between the two.

Another theme that is explored is that of choice and consequence and it is examined mainly through the way that Okonkwo's choices lead to other choices that affect his fate.

Another theme that comes up in the novel is change and in particular the cultural changes that occur in the tribe with the arrival of the white missionaries.