Quite honestly, you can tell a lot from the full title of the book which is Green Shadows, White Whale: A Novel of Ray Bradbury's Adventures Making Moby Dick with John Huston in Ireland. This tells us that there are two main characters in the story: Ray Bradbury and John Huston. (Yes, there are minor characters such as various jolly Irishmen like Doone and Huston's wife, Ricki, but they are not really important to the main plot and theme.) Ray Bradbury, apart from being the actual author of the work, is the writer en route to Ireland determined to pen a screenplay for an adaptation of Melville's Moby Dick. John Huston is the one who has created that adaptation. Huston is also the director who is quite intimidating, eccentric, erratic, and almost crazy.
The theme is the (this time) humorous hunt for the elusive "white whale." In Melville's book it is a serious theme of obsession that overcomes Ahab, who is determined to chase the white whale to the depths of the sea and even to his own death. However, in Bradbury's book, the account is very humorous and fraught with interesting cultural (and often drunken) humor as well as with the crazy arguments and conversations that take place between writer and director. This time the "hunt" for the "white whale" is Bradbury's hunt for the correct screenplay adaptation.
Sure, it’s money runs the world ... But it is music that holds down the friction.
There are questions about everything in writing the screenplay, from whether to include music (see quote above) to who to cast for the main parts. The entire thing is done with great dark humor.
Here we should mention a secondary theme which is the American idea of Irish culture. This is where most of the humor enters the novel.
Nearby, an old man was similarly engaged in finding the pattern of his life in the depths of his glass.
Yes, it is dark humor, but it is humor nonetheless. There is quite a bit to be said about the Irish culture if you read between the lines of this novel. Remember, it is not written through Irish eyes, so all of the errors and misconceptions are wide open for us to laugh about.