Translation of "Theme" in Chinese

What is the theme of his latest novel?

他最后一本小说的主题是什么？

The principal theme of the book is the American Revolution.

这本书的主题是美国革命。

In England, parks taking rare breeds of animals as their theme are becoming increasingly popular with people.

在英国，以珍稀物种为主题的公园变得越来越受人们的欢迎。