What is the thesis statement of the book The Night Circus?

I am torn on how to interpret your question.  On one hand, I think that you might be asking what a major theme to the book is.  On the other hand, I think that you might be asking what is a possible thesis that you could write about the book.  I will try to answer the question in both ways.  

Thematically, I think the book has several identifiable themes.  One of the most important themes is the theme of time.  Time is super important to The Night Circus.  In an interesting twist though, time is not that important to the people performing in the circus.  That's because as long as Celia and Marco are involved in their "battle," the circus and performers will not age. The theme of time has a symbolic object in the novel as well.  The object is Herr Thiessen's clock.  The clock works of course, but it seems to dilate time as well.  

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.

Another theme is the theme of light and dark.  The circus only performing at night is an easy example of this theme, but the theme is much deeper than that too.  Like the theme of time, the book uses physical objects to further cement this theme into the mind of a reader.  To help with the theme of light and dark is the color palate of the circus itself.  It is done entirely in black and white.  Even costumes are based on that color scheme.  The twins, Widget and Poppet, use the color interplay in their act alongside their costumes as well.  

"And how much you like whoever you're stuck there with," Poppet adds, kicking his black boot with her white one.”

A final theme is the theme of love.  Marco and Celia are battling each other at the command of their masters, but they are deeply in love with each other.

“Everything I have done, every change I have made to that circus, every impossible feat and astounding sight, I have done for her.”

 They are desperate to find a way to be together, to keep the circus in tact, and not to play their "game" to its intended finale (death).  If I had to suggest a single "thesis" about the main point of the book, it would be a statement about how love is able to conquer all obstacles.  

If you are looking to write a thesis statement about the book, my recommendation is to write a thesis that discusses the book's themes, its imagery, or its characters.  I always found Alexander an interesting character.  He exists outside of time like the circus performers, but he isn't a part of the circus.  He exists outside of it.  He also exists outside of the book's color palate as well, because he only wears gray.  He's neither black, nor white.  Your thesis could be something like "Alexander is critical to understanding the importance of the circus, because he alone manages an emotional distance from the circus and its performers."