One of the main themes (especially when viewed through a modern critical lens) in Moby Dick is race and racism. The harpooners, Queequeg (from the South Seas), Tashtego (a Native American from Martha's Vineyard), and Daggoo (from Africa), are all people of color. They do the hard and dangerous work of harpooning whales, and they are the first people to confront the whale. However, they do not receive credit for what they do. For example, Tashtego is the harpooner who slays the first whale killed in the book, but Stubb, the white second mate on the Pequod, is credited with killing the whale. The other mates on the ship, Starbuck, Flask, and the captain, Ahab, are also white. They pursue the glory of killing whales and receive credit for it, while the harpooners, people of color, do the most dangerous work.

The whiteness of the whale is also significant, as the whale itself signifies what everyone desires. The fact that what everyone wants is white can be understood as an implicit valuation of the color white. In turn, this means that whiteness, and, by extension, white people eclipse the people of color around them. All anyone can see is whiteness—darkness and people of color are invisible. 

Herman Melville dedicated his novel, Moby Dick, to Nathaniel Hawthorne and wrote him, "I have written a wicked book, and feel spotless as the lamb."  While there are several major themes in Melville's great work, perhaps the central theme is that of the individual in conflict with nature which brings into play Religion and God's role in the natural world.

Melville marked repeatedly verses from the book of Job, such as the verse in the fourteenth chapter when Job asks his despairing question about a future life, "But man dieth and wasteth away: yea, man giveth up the ghost, but where is he?"  Certainly, the implications here of the white whale as a metaphor for the forces of nature and fate are apparent. 

In the beginning of the novel, Father Mapple gives a sermon that reflects the contemporary religious attitudes of the early nineteenth century Protestantism.  On the voyage, Starbuck reflects these attitudes as well and conflicts with Ahab who vows to fight the "inscrutable malice" of the whale and break through the "pasteboard mask" of all visible objects.  That is, Ahab defies conventional attitudes and fights against the Calvinistic sense of fate and "Innate Depravity."  Ahab refuses to resign himself to the predestination of divine providence. Melville, like his contemporary, Nathaniel Hawthorne, felt very much the "Puritanical gloom" of his times, and as a Anti-Romantic, he also felt the dark forces of nature, forces that lie at the bottom of the sea while the good, perhaps, is on the shore and in the sky. 

What are some major conflicts in Moby Dick?

The largest conflict is between Ahab (and, to some extent, his crew) and the whale Moby-Dick.  While this is the central conflict that drives the novel, there is also a persistent conflict between Ahab and the crew over the ship's mission.  For example, Chapter 54, "The Town Ho's Story," a sailor from another ship tells a story about a group of sailors rebelling, or conducting a mutiny, against their captain.  Furthermore, much later, in Chapter 123, "The Musket," his first mate Starbuck picks up a musket in Ahab's cabin while the captain is sleeping and thinks,

I stand alone here upon an open sea, with two oceans an a whole continent between me and law. --Aye, aye, 'tis so.--Is heaven a murderer when its lightning strikes a would-be murderer in his bed, tindering sheets and skin together?--And would I be a murderer, then, if"--and slowly, stealthily, and half sideways looking, he placed the loaded musket's end against the door.

From this interior monologue, the reader can tell that Starbuck is contemplating killing Ahab and, thus, overthrowing his authority.  Because they are so far out at sea, he imagines that he could get away with it, but ultimately he does not kill Ahab or begin a mutiny.  One of the main ambiguities of the novel is the status of mutiny; it is never really clear why the crew does not overthrow their rather tyrannical captain.

What are the main conflicts and the theme/goal of Herman Melville's Moby Dick?

As a classic, Herman Melville's "Moby Dick" can be read on different levels.  On one level, it is a narrative about whaling and whales and one obsessed sea captain who seeks revenge again a levithian called Moby Dick who cost him his leg in a previous encounter.  This narrative is set in Nantucket, Massachusetts, a site renowned for its craggy seamen and fearless whalers, and is narrated by a young man who signed on the the enigmatic Ahab's ship in order to be gone "for a year or two."

But, since the narrative of the chase for Moby Dick does not occupy more than a fourth of the novel, Melville's work entails much more than the story of a maniac sea captain.  It is, rather, a metaphysical examination of the essence of nature.  For, as Captain Ahab declares early in the novel, what man sees of nature is but an inscrutable "pasteboard mask."  And, Ahab tells his crew that he would break through this mask in order to understand the meaning of existence behind it.  As what is termed a Dark Romantic, Melville and others like him perceived nature as a force that often works against man.  Moby Dick is the "pasteboard mask" that conceals the malevolent force against man, particularly Ahab who desires a rencounter with the "levithian" so that he may "break through that mask."  For, Ahab believes that if he can capture the great white whale, he can conquer the malevolent force against him.

Having remarked that the sea was his school, Melville's novel encompasses several themes, but the overriding one is man's search for meaning in his existence against the dark forces of nature, with the voyage of the Pequod being a metaphor for life.  In Chapter 58, Melville writes,

As the appalling surrounds the verdant land, so in the soul of man there lies one Tahiti, full of peace and joy, but encompassed by all the horrors of the half-known life.  God keep thee!  Push not off from that isle, for thou canst never return.

Once someone has encountered evil--has left the Eden-like isle of innocence and has learned of life--he can never go back to his naivete, but must seek answers as Captain Ahab has felt compelled to do, risking his very life. himself.

What are the main conflicts and the theme/goal of Herman Melville's Moby Dick?

Moby Dick is considered to be Herman Melville's masterpiece.  It was largely influenced by the fact that Melville, himself, worked on a whaling ship for awhile.  Moby Dick takes place in the early 1800s--probably around the 1830s or 1840s.  The majority of the novel takes place on the ship, the Pequod.  Ishmael, the novel's narrator, goes to Nantucket in order to board the ship.  Nantucket was considered to be the whaling capital during that time period.  From there, the Pequod starts its voyage to the southern tip of Africa.  Its goal is the same as the ship's captain's (Ahab): to find and kill Moby Dick, the great white whale that has become a legend during this time.  The problem is that Ahab becomes so focused on killing this legendary whale that he is unable to focus on anything else.  He goes to great lengths and risks the lives of his crew in order to find and kill Moby Dick.  Ahab's monomania is the chief conflict of the novel.

Conflict: Man v. Nature: Ahab's quest to kill the great white whale

Theme: I have attached a list of themes from enotes.com for you to look at.