You have innocence and truth to consider too, but I think a major theme of To Kill a Mockingbird is perspective. You might even couple that with selflessness. My image of Harper Lee is that of a woman who must have cared about other people's problems enough to try to make a society think about acting the same way.

By chapter 31, Scout indeed walks Boo home and that is referenced by the other editors. She looks out from his porch and imagines the world through his eyes, from his perspective. It makes her childhood actions seem to her for the first time... childish. She is not afraid of him anymore, but because she understands his perspective for the first time.

What always stands out to me in Chapter 31 is Scout's bedtime discussion with Atticus about the book The Gray Ghost. I think that title alone is reflective of Boo Radley's phantom status that the kids had of him. What Scout says about the book is the most important. She notices that there was someone who was messing wiht the clubhouse, and everyone thought it was stoner's boy, but it wasn't him at all... in fact he was really nice. This reflects every innocent character in the book... Boo, Tom, and mockingbirds. And all it took was looking at it from a different perspective to see that it wasn't stoner's boy after all.

Re-read the last two pages, you'll get some really good quotes.

In To Kill a Mockingbird by Harper Lee, the sympathy motif is introduced in Chapter 1 when Atticus instructs Scout

'You never really understand a person until you consider things from his point of view-....until you climb into his skin and walk around in it.'

Now, in the final chapter this motif comes to fruition as Scout, as she stands on the porch of Boo Radley and surveys the neighborhood from his point of view, arrives at an understanding of the reclusive "mockingbird" that is Boo Radley as a man like any other man:

Atticus was right.  One time he said you never really know a man until you stand in his shoes and walk around in them.  Just standing on the Radley porch was enough....I had never seen our neighborhood from this angle.

She also understands how much Boo has done for her and Jem and Dill, while at the same time they have not reciprocated:

We never put back into the tree what we took out of it:  we had given him nothing, and it made me sad.

This remark touches upon the mockingbird motif which acts as a device by which the two plot elements are unified.  For, the first part of the novel and the Boo Radley mystery, parallels the second part which is concerned with the Tom Robinson trial. Harmless members of society, both of these characters can be viewed as a mockingbird; for, while both are innocent people, they both are persecuted by society.

And, as Scout recalls that she feels very old, and there "wasn't much else for us to learn," the novel ends with the maturation of Scout, thus defining To Kill a Mockingbird as a bildungsroman, or novel of maturation.  For, Scout and Jem have come to understand why their father has taught them what he has, as well as why their father has chosen certain courses of action.

Another motif present in Chapter 31 that is tied to the first part of the novel is the recurring idea of education.  In Chapters 1 and 2 the reader understands that the education that Atticus gives his children surpasses that of the rigid classroom.  They learn much from Atticus--humility, fortitude, honesty, fairness; they learn that simple observation of human nature brings great knowledge. 

With Scout and Jem's new knowledge comes the end of their superstitions and fears.  As Scout and Jem have learned more about their world their fear of "haints" has disappeared as well as their fear of Boo Radley.  (The bildingsroman theme is also here.)

Clearly, the final chapter ties together the two parts of the novel as well as underscoring certain motifs and themes.

To me, the major theme of this book is tolerance.  The book emphasizes that we should tolerate others who are poorer than us, who are of a different race, who are old and weak and who are just weird or scary.  I think this is why it continues to get assigned so much.

The last chapter wraps this theme up nicely.  Practically the last words in the book are Atticus telling Scout that most people are nice once you start to see them.  This chapter also sees Scout coming to completely understand Boo -- she understands his body language, and she understands that she should not let people see her leading him home (instead, he should look like he is escorting her).

So the book ends with tolerance of Boo -- the very figure who was so feared and mocked early in the story.  This neatly illustrates the book's major theme.