The central theme of "How to Catch a Monster" is about facing and overcoming fears. The young ninja's journey into the closet to confront the monster symbolizes the courage it takes to face the unknown. This theme is particularly relevant for children who might be dealing with their own fears, whether it's starting school or sleeping in the dark.

Another significant theme is the idea that friends can be found in the most unexpected places. The young ninja's discovery that the monster is not scary but rather a potential friend teaches children about the importance of not judging others based on appearances and being open to new friendships.

The book also celebrates the power of imagination and creativity. The young ninja's inventive approach to catching the monster and the whimsical illustrations encourage children to use their creativity to solve problems and see the world in a fun and imaginative way.