What are the main themes in the book of Genesis?

The main theme of Genesis is, as its name suggests, origins. The first book of the Bible provides us with an account of the origins of the universe, life on earth, humankind, and sin. It thus accounts for the fallen state of the world and of humankind's need for a redeemer. The precise identity of this redeemer isn't spelled out in Genesis, but of course we know that it refers to Jesus Christ, the only begotten son of God.
In the very first book of the Bible, then, we can see the thematic continuity between the Old and New Testaments. We can also understand why, in orthodox Christianity, the New Testament is seen as the fulfillment of prophecies made in the Old Testament. What is begun in the Old Testament is completed in the New with the coming of Christ.