I would say that you could find many themes in the novel.  One such idea could be the importance of memory.  The notion of recollecting plays a vital role in the novel.  On one hand, Hannah starts off disliking her ethnic identity and does not see the importance of it.  In many ways, she seeks to forget that she is Jewish and hopes to alleviate such a burden through a superficial lifestyle that does not acknowledge what it means to have memory.  In seeking to forget it, she actually plunges into a world that she does not know, lacks understanding, yet it is a world that seeks to erase her and her people from its memory.  The fact that she, as Chaya, does not "remember" her aunt and uncle is another example of memory.  Her experiences in Poland are ones where the curse of memory are nearly impossible to overcome.

Before you begin to answer this question you must first remember:

-  Theme is a subject or main idea of a work of literature.
-  Theme is NOT plot summary.
-  Theme statements do not give advice (they do not contain you, should, ought, etc.)
-  Theme statements should be supported by text throughout the work, not just one section.

That said, here are some notes about how to write a theme statement for ANY piece of literature.

1.  Make a list of SUBJECTS covered in the text (not plot details but things like, relationships, fighting, growing up, etc.)
2.  Choose one, or two that seem to relate to each other.
3.  Ask, "What is the author trying to say about (subject)?  *I like to put things through what I call the "cause-effect" machine.  Make a list of all the things that either caused the subject or the effects of the subject.  You use plot here, but by examining the patterns, you can often answer the bigger question: What is the author trying to say about (subject)?
4.  Answer the question in #3 with a complete sentence.  This is your theme statement.  Using words like causes or results in are often helpful.
5.  Prove your theme statement with examples from the plot.

A final word:

The key to theme statements is to make them broad enough to be universally applicable (ie. to other works of literature or perhaps even life itself) but not SO BROAD that they could be virtually applied to absolutely any work of literature.

For example (Think of Romeo and Juliet):

1.  Fighting always leads to death.  (Too narrow in one sense, but too broad when you consider - what kind of fighting?)
2.  Fighting can be a negative thing.  (Um, yes, duh.  Obviously.  Way too broad.)
3.  Hasty decisions often lead to unnecessary fighting, which can be disasterous.  (Just narrow enough, but still universally applicable.)