What is the theme in the book Tuck Everlasting?

There isn't a single theme for this book.  Rather there are multiple themes.  

One important theme is the theme of life and death.  The Tucks are all immortal, and Winnie is given the chance to join them.  Jesse Tuck thinks it is great fun to be immortal, while Angus Tuck desperately wishes for the chance to die again.  To Angus, he isn't truly living if he can't die.  

"You can't have living without dying. So you can't call it living, what we got. We just are, we just be, like rocks beside the road."

I think that love is a big theme in the book as well.  Jesse and Winnie seem to have a budding romance.  More importantly, there is a large amount of familial love in the book.  It is clear that Winnie is a much loved girl.  Her family loves her dearly.  The Tucks love her dearly, and she loves them back.  

Now, remembering the visits of the night before, she smiled—and found that she loved them, this most peculiar family. They were her friends, after all. And hers alone.

Those feelings of love are powerful motivators for the characters in the novel.  Mae is motivated to kill in order to protect Winnie, and Winnie is committed to breaking the law in order to see Mae set free.  

There is a theme of choices throughout the book.  Winnie chooses to run away.  She chooses to not accuse the Tucks of kidnapping.  She has a choice about whether or not to drink the spring water.  She chooses to help the Tuck family free Mae. 

I believe that there is also a theme of freedom present within the book.  When the book begins, Winnie feels imprisoned by her family.  She longs to have more freedom and independence.  It's why she tries so hard to motivate herself to run away.  The Tuck family gives her that freedom, because they do not try to micromanage her.