One of the themes of the novel is the power and truth of nature and pagan religions over Christianity. At the beginning of the book, Tess is dancing in a May Day festival, and Hardy writes:

"The banded ones were all dressed in white gowns--a gay survival from Old Style days, when cheerfulness and May-time were synonyms--days before the habit of taking long views had reduced emotions to a monotonous average."

There is something fresh and earthly about Tess, as she comes from a culture with long associations with the earth and the joyousness of paganism. Hardy implies that the end of paganism brought about a reduction in joy and the human spirit.

Hardy also implies that organized religion is corrupt for cursing Tess, who has been raped by Alec. When her out-of-wedlock child is about to die, Tess baptizes him herself. Hardy writes:

"Tess then stood erect with the infant on her arm beside the basin; the next sister held the prayer-book open before her, as the clerk at church held it before the parson; and thus the girl set about baptizing her child."

While this isn't a real baptism, as it's not conducted in a church by a minister, Hardy suggests that Tess, standing noble and erect, is holier and more connected with divinity than is the religion that has cursed her.

At the end of the novel, Tess stretches out and falls asleep at Stonehenge while waiting with Angel for the authorities to find her. She says to Angel, "And you used to say at Talbothays that I was a heathen. So now I am at home.” It's almost as if she is being sacrificed on a heathen altar for the crimes she has committed. Her crimes are considered unchristian, yet there is something pure and noble about her, as her crimes are understandable. She has been raped, and though she is a victim, she has been vilified for it. Hardy suggests that paganism is purer than Christianity.

Another theme in the novel is that of knowledge versus ignorance. Tess and Angel struggle with their parent's unwillingness to accept change and progress and, therefore , this causes a lot of friction between them.

Tess, who has had formal schooling, is not only in possession of a greater intellect than her mother, but also has a much better sense of right and wrong.

Angel is different, because, with the exception of himself,  he is in a family of scholars. Angel has common sense, and is able to see that for all his family's "real" education, they are not always wise in their choices.

Both Angel and Tess see their parent's as choosing to be ignorant, or at the very least, unwilling to move with the times, and their relationships suffer for it.

There are a few different themes, but if you had to pick just one, I would say: Fate and Chance.

So much of the novel revolves around the idea of fate, chance, etc. and what that might mean to life in general. Most of the events are set in motion by fate, and then change due to chance.