I have to write a book review on Their Eyes Were Watching God, but I do not know what would be considered the main theme of the book.
Just remember that a book review must not exclusively talk about theme. You must also mention other elements, such as plot, characters and setting, in addition to your own thoughts about the novel.
However, certainly the central theme seems to revolve around Janie's "journey" through life and in particular her growth in her own understanding of herself and a strong love of her own independence. This central movement of Janie towards wisdom and maturity occurs as a response to the different relationships that she has. The novel seems to suggest that fulfilling relationships can produce growth, but only when they are based on equality, as Janie's relationship with Tea Cake demonstrates.
If we view the novel as being about Janie's journey towards self-fulfillment, we can see that she manages to escape from her relationships with Logan and Jody that hinder this goal but does not do this with her relationship with Tea Cake. We see that in this relationship, Janie establishes her true identity and enjoys independence. Through this relationship, Janie establishes an almost spiritual connection with the world and even feels that Tea Cake, although he is dead, is still with her in some way.
You might like to consider the following quote from the final chapter as part of the consideration of this theme:
Two things everybody's got tuh do fuh theyselves. They got tuh go tuh God, and they got tuh find out about livin' fuh themselves.
Janie has done both of these things in her quest for self-enlightenment, which is something that distinguishes her from the ignorant and gossipy villagers she returns to. Having done it, she has reached a position of security in her own identity and independence in who she is.
This question has been answered many times here on eNotes. Here is a link for you: http://www.enotes.com/their-eyes/q-and-a/what-main-theme-message-novel-their-eyes-were-153691