What are the main themes in The Mayor of Casterbridge by Thomas Hardy?

The most important theme in The Mayor of Casterbridge is that of blind Fate.  For Hardy, Fate is blind, arbitrary and merciless and always brings misery, pain, sorrow and suffering. There is no fate that is a good fate. Fate is delivered through coincidences and circumstances that cannot be foreseen. Once an opening allows for the working of fate's designs, there is little or nothing that can be done to stop its inexorable flow to a catastrophic end.

In The Mayor of Casterbridge, coincidences and unforeseen circumstances work in concert with Micheal Henchard's tragic character flaws, and fate is loosed upon him and will push him to his miserable end regardless of what he does or does not do. Once Michael yields to the choice to sell Susan and the baby despite all the protestations and efforts to bring him to his reason, his fateful end is sealed. This leads to the twin theme of how personal choice interacts with Fate.

Hardy explores how, and whether, fate can altered, forestalled, or avoided by personal choices. He explores how personal choice may or may not influence the outcome of a person's life. He answers the questions: May a person have a happy end instead of a miserable end if by some means Fate can be contained or avoided? And if so, by what means?

Henchard illustrates how a person may unleash the forces of fate through immoral, rash, unethical, cruel choices. He shows how fate is loosed in all its final fury by choices that are unreasoned, unsound, and unseeing: no foresight is employed to calculate future effects of present choices. Farfrae illustrates the opposite. He shows how calm well-ordered thought and choices can lead to a smooth course in life by forestalling any opportunity for Fate to unleash calamity that dooms life to a miserable end.

Hardy's answer to his own questions is that a person may by some means avoid the misery of Fate and that means is cool, calm, reserved, careful, deliberate well-reasoned thought that foresees the future consequences of present choices.