As you probably know, each book can have a number of themes and Don't Look Behind You is no different. I am going to suggest a couple more here. For this book, these two are my favorite: fear & family.
This book thrives on suspense and is about the witness protection program; therefore, fear must be a theme! Note how the program is presented:
The world as we knew it ended for us on a Tuesday afternoon in May.
Just look at how the things requested of the two children breed fear. First, Bram is suddenly forced to wear contacts in order to hide the distinct coloration of one of his eyes. Second, April is forced to give up her sport of choice (tennis) because she is a "redhot tennis player" and this skill will call attention to herself. Adults aside, whenever children are brought into the fray by muting who they are, one can be sure that fear is behind all involved.
Further, it is precisely the theme of family that is able to get the main characters past their fear and, as such, is the more important theme here. Unfortunately, first, the characters get to a point of question who they really are anymore since so many things about them have to be changed. However, they realize that it is their family unit that can keep them strong. Through family (and ONLY through family, not through the government protection) they can support each other through all of the changes.