What are the recurring themes throughout the novel The Night Circus?
As the previous answer stated, time is a major theme that runs throughout the book. Another theme that is central to the story is love. The Night Circus depicts a couple of different kinds of love, so a reader shouldn't expect it to be only about a boy and a girl that fall in love.
For example, a great deal of the story is about Bailey and his love for the circus. From the first time that he sees the circus tent, he is immediately drawn to it by some all-consuming force. He sneaks in and meets Poppet and Widget, and from there his soul becomes more and more entwined with the beauty of the circus and the beauty of all its performers. His love for the circus is so deep that he willingly takes over the circus, and since the circus is magically timeless, he is likely to be the caretaker forever.
Celia and Marco illustrate the standard romantic love theme. Theirs is a powerful love story, and it depicts the standard mythical belief that love is the most powerful force on Earth, it is forever, and no problem can stand in the way of true love. Celia are Marco are supposed to be enemies in an epic battle of magic that will end when one of them dies; however, they fall in love. Their love for each other allows them to begin looking for a way to get out of an impossible situation. They are able to save the circus, save their own lives, get out of the magic competition, and be happily in love together forever.
One of the recurring themes in this novel is that of time. Time is something that is vitally important to the novel as indicated by the powerful symbol of Herr Thiessen's clock, which signifies the way that time has stopped still, or at least is only moving very very slowly, for the purposes of the circus and those involved in it. Note the description of the clock and what happens as it begins to change:
Meanwhile, bits of the body of the clock expand and contract, like pieces of a puzzle. As though the clock is falling apart, slowly and gracefully.
All of this takes hours.
The time taken by the clock to fully open up and the magical scene that occurs within it with the various figures performing their actions indicates the magic that is at the heart of the night circus, and at the same time the fact that the clock "takes hours" to perform its action signifies the way that the circus conflates and protracts time, slowing everything down. This is of course why those involved in the circus do not age or die, and continue in a kind of stasis, staying the same whilst they move around the world on their itinerant journey in the circus. Such magic can only occur out of reality where the normal rules of life, such as time, are suspended.