Some of the themes in the book are Good vs. Evil, Fate, Death, and Friendship. Good vs. Evil is the main theme, as the characters struggle throughout the book to fight evil for a worthy cause they believe in. Bilbo and Gandalf are two of the main characters fighting for good, while many evil creatures like dragons and goblins cause destruction and suffering. However, the concept of good and evil is blurred at times in Bilbo's journey, and this leads to much learning for characters.
The concept of fate and free will is also explored in the novel. There are prophesies that have foretold of events that come about in Bilbo's adventures. The theme of death is also very prevalent in The Hobbit, and the characters struggle to survive the dangers they encounter along their journey. Friendship is another theme; sometimes friendship is gained easily, but other times in the novel friendship must be earned. At first Bilbo is at odds with the dwarves, but he comes to learn that they can be loyal friends.
Enotes has an excellent study guide for this novel. The link for the themes page is attached below for more in-depth information.