The theme? There isn't one. There are quite a few themes in the book Tuck Everlasting. It's actually kind of amazing how many themes are packed into such a short book. It's only 139 pages long, and in those 139 pages can be found themes on life, death, time, choices, love, and friendship.
The theme of love is demonstrated in a few ways. There is a budding romance between Jesse and Winnie for sure, but there are other examples of non-romantic love too. For example it's clear that the Tuck family loves each other and loves Winnie as one of their own. It's also obvious that Winnie loves the Tuck family back. It's why she is willing to help out with the escape near the end of the book.
Remember that a main plot of the book is whether or not Winnie is going to choose to drink the spring water or not. It's her choice, but it's not an easy choice, because it forces her to wrestle with very real questions about life and death.
One of the driving themes of Tuck Everlasting is the idea of immortality and time, which is in every single chapter of the book. The Tuck family have been frozen in time and will never die. They are stuck at the edge of the river while the river continues to flow around them. Winnie Foster must eventually choose whether she wants to give up the natural order of life and drink from the spring, becoming immortal. But she ultimately decides that there is nothing to live for if you never run the risk of dying. It is the possibility of loss that makes life precious.
Other related themes are the idea of freedom from boundaries, and changes that happen in life. Winnie lives in a tightly-protected world where she is sheltered and overly protected. The Tucks symbolize a freedom for her, a freedom from the strict rules of her parents and a freedom from time itself. In some ways the Tucks live a loose life, traveling around from place to place, not staying anywhere too long in case someone recognizes them. However, there is some fear in their lives, too, fear of discovery and fear of what people would do with the water from the spring if they knew. Winnie is afraid of many changes that are happening or will happen in her life, but as she witnesses the Tucks trapped in their bodies, never growing older or changing, she realizes that changes are a part of life and are not a bad thing at all.