The underlying theme of Black Beauty—i.e. a theme that's not explicitly articulated—is the fundamental unity of every living thing. In writing the book, Anna Sewell undoubtedly wanted to draw the attention of Victorian England to the appalling state in which many animals were kept. But underlying this was a firm belief in the interconnectedness of every living thing, man and animal, flora and fauna.

It is only by seeing ourselves as part of a much bigger whole that we can begin to develop the kind of lasting empathy for animals that Sewell wants us to feel. It's not enough to read about the many examples of animal cruelty and exploitation in the book and say "That's terrible! We must do something." What we need to do is to change our whole attitude towards animals once and for all. And that can only be done if we join with Sewell in seeing ourselves as related to our fellow creatures as part of a vast cosmic unity.

The theme is the main topic, idea, or message in a piece of writing.

The theme of Black Beauty is most definitely that horses (as well as all animals and also people) ought to be treated kindly, ethically and humanely.

The book is about the life story and experiences of a horse called Black Beauty who is treated well by his owner in the beginning of his life but is later mistreated by other owners. The story describes how horses were often treated badly in Victorian England and how the animals suffered injury and sometimes even death due to the mistreatment.

The book also underlines issues of social class, inequality and wide differences between the poor and rich in England at that time. The book was so influential after it was published in 1877 that it even contributed to the eradication of the bearing rein. This was a rein used to pull horses' heads back, causing them pain.

The theme of Anna Sewell's Black Beauty is primarily the treatment of horses and secondarily the treatment of other animals. Anna Sewell herself was an activist, extremely concerned with humane treatment of horses and wrote the book from the point of view of the horse to arouse in her readers compassion for animals. She was specifically opposed to the bearing rein, which she thought especially cruel because it held the horses’s neck in a very awkward position.. She also was an active member of the temperance movement and alcohol abuse appears as a secondary theme in the novel.

The main theme of Black Beauty is the care of animals, particularly horses.  Many people have said that Sewell could have written a non-fiction text describing the care of horses instead of Black Beauty because there is so much in it on how to care for them.  In this care, Sewell also alerts the reader to the abuse of animals.  The reader wanted to gain sympathy from her readers for these animals in an attempt to help end the abuses.

One other theme would be morality and righteousness.  Sewell was raised as a Quaker and therefore had a strict personal code of conduct.  This strict behavior is seen throughout the work. 

What is a theme in the chapter "My Breaking In" from Black Beauty?

The chapter titled "My Breaking In" is chapter 3 of Black Beauty. The chapter explores a number of themes that are developed throughout the book, and one such theme is courage.

Throughout the book, Beauty is confronted with a number of things that scare him, but he always courageously meets those challenges. While "My Breaking In" doesn't put anything life threatening before Beauty, he is experiencing things that he has never experienced before, and they are uncomfortable things. Beauty's master even says to Beauty that the point of the breaking in is to make sure that Beauty isn't frightened.

Personally, when someone tells me I am going to experience something in order to make it not scary, that makes me nervous and a bit fearful. Beauty doesn't shy away from the upcoming discomfort. He courageously learns to deal with how uncomfortable things like a bit and harness can be. Beauty describes in detail the entire process, and we realize that his master intentionally puts Beauty in situations meant to scare Beauty. For example, Beauty is put in a field near passing trains. The goal is to desensitize Beauty to the scary machinery. What's wonderful to watch is that Beauty steps into each new challenge courageously rather than with massive amounts of timid trepidation. Beauty knows that the training will be good for him, and he trusts his master, so he bravely moves through the uncomfortable training period.