In Milton's Paradise Lost he is writing his own Genesis story. He is describing th fall of man or humankind and the fall of the devil or Lucifer (and his angels). In the poem it shows the rebel angels revolting and then how they are thrust into hell. It shows the creation of the world, along with Adam and Eve and their inability to listen to and obey god's one request. Of course they were also thrust out of the garden of Eden.
I guess you could call the theme: the genesis story or seeking justice because Milton is explaining why the humans or mankind was banished.