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.