What is the theme in The Cat Who Turned on and Off by Lilian Jackson Braun?

Truth and illusion is the main theme. As with most with most murder mysteries, the tension between truth and illusion is the book's primary theme. On the most apparent level, the theme is explored through Qwilleran's search for the truth buried beneath the illusions of lies and alibis, mysteries and denials, put forward by the other characters in order to protect themselves from suspicion. In other words, plot and theme are intertwined with the former dramatizing the latter and the latter motivating the former.