There are three major themes in Hardy's novel: unrequited love, catastrophe, and social hierarchy. The following is excerpt from eNotes. You can find much more detailed information on each theme by visiting the link below, but here is a sampling:
Unrequited love: Much of the plot of Far from the Madding Crowd depends on unrequited love—love by one person for another that is not mutual in that the other person does not feel love in return. The novel is driven, from the first few chapters, by Gabriel Oak’s love for Bathsheba. Once he has lost his farm, he is free to wander anywhere in search of work, but he heads to Weatherbury because it is in the direction that Bathsheba has gone. This move leads to Oak’s employment at Bathsheba’s farm, where he patiently consoles her in her troubles and supports her in tending the farm, with no sign he will ever have his love returned.
Catastrophe: This novel focuses on the way that catastrophe can occur at any time, threatening to change lives. The most obvious example occurs when Oak’s flock of sheep is destroyed by an unlikely confluence of circumstances, including an inexperienced sheep dog, a rotted rail, and a chalk pit that happens to have been dug adjacent to his land. In one night, Oak’s future as an independent farmer is destroyed, and he ends up begging just to secure the diminished position of a shepherd.
Social Hierarchy: This novel offers modern readers a clear picture of how important social position was in England in the nineteenth century and of the opportunities that existed to change class, in either direction.