I would say that from Chapter 10 onwards, which charts Amir and Baba's terrifying escape from Afghanistan, one of the central themes that emerges is the way that you must face up to and challenge acts of injustice. Note how Baba does this as he stands up to the Russian soldier who wants to rape the Afghan woman who they are travelling with. When this happens, Amir is judged and found wanting as he automatically things back to another time when he stood by and did absolutely nothing in the face of an act of gross violence and abuse:
"Tell him I'll take a thousand of his bullets before I let this indecency take place," Baba said. My mind flashed to that winter day six years ago. Me, peering around the corner in the alley. Kamal and Wali holding Hassan down. Assef's buttock muscles clencing and unclenching, his hips thrusting back and forth. Some hero I had been, fretting about the kite. Sometimes, I too wondered if I was really Baba's son.
Note how the contrast is introduced. This of course is a theme that dominates the entire novel as Amir is moved into a position where he is unable to ignore injustice and violence and has to take a stand, just like his father does here at this point in the novel. Throughout the novel, Amir compares himself unfavourably to his father as he exhibits the kind of moral cowardice that Baba scorns.
I just finished a semester of teaching Middle Eastern translation literature, so this question is relevant and of-the-moment.
Hosseini plays with quite a few themes throughout his novel, some of which are to introduce us to Afghan history and culture through a Western lens (meaning a history Westerners can understand).
Starting in Chapter Ten, we've basically sprinted forward in Amir's life--because those sixyears are inconsequential towards the achievement of Amir's growth which he has yet to do. You'll do the same thing in Chapter 14, when Rahim Khan phones and reminds Amir that, "there is a way to be good again." It's the challenge to admit to past mistakes and essentially 'man up.'
You'll find throughout the story that Hosseini plays w/intertexuality (Sorah= part Oedipus, part Iliad, part Achilles) and we certainly gain an understanding of the Afghan people and their culture (pre/post Taliban) through the history and liberal sprinkling of Afghan words. Take for example this quote from Chapter Ten:
You couldn't trust anyone in Kabul...for a fee or under threat, people told on each other, neighbor on neighbor, child on parent, brother on brother.
Imagine what life would be like if you couldn't trust anyone. Anyone! That said, even after the move to America there is still an issue of trust, Consider this example from Baba trying to purchase items at a grocery store.
What kind of country is this? No one trusts anybody!
The theme of trust continues, but more important, we see that Baba isn't adjusting to his new life, while Amir embraces the chance to forget:
America was different. America was a river, roaring along, unmindful of the past.
Amir can reinvent himself and try to allay the memories of Hasaan that continue to haunt him. More American than Afghan, Amir tries to embrace his new American way of life, but finds that at every turn he is reminded of tradition, culture and the Afghan way.
We also learn that the Afghans have a long history of war (Russian invasion predates the Taliban...and Americans) and that really, war is a way of life and to escape it means fleeing. To survive, the Afghan people forge a new community, complete with the old traditions, in America. So even though they're "unhomed' (a post-colonial theory term), they are able to bond and salvage their traditions and sense of community.
You see again that sense of tradition in the interplay with Amir and Soraya. Amir is unable to start up a direct conversation with her because tradition matters, and even though Soraya is a 'fallen woman,' duty, honor, pride, and tradition are huge factors. So, the underlying message is to embrace culture, no matter where you end up.
An equally prevalent and important theme is honor and pride--traits tantamount to an Afghan man and, as you've already seen, Amir is not capable of honor...yet. Baba, who seems to be the most heroic character (at least to Amir) is tainted, too. There is also the recurring theme of Amir's search for Baba's love.
I loveteaching this book because Hosseini is brilliant in how much culture he injects. In just these five short chapters go back and see how much you've learned of Afghanistan and her people.
Even though I know this book well, I recommend taking a look at the e-notes Study Guides and the politics/culture link below for some different approaches.