Every episode has an opening and ending.
The tone of these are based off of the tone of the chapter.

Example:
The first chapter has a more generic tone to it and has generic and happy opening and ending themes.

Chapter 1 has a generic theme.
Chapter 2 will have a more serious theme.
Chapter 3 will have an upbeat theme.
Chapter 4 will have a suspensful theme.
Chapter 5 will have a sad theme.

The series gets more and more darker as it nears the end.