This theme is universal because people can use it in their life to remember not to judge each other on what they are on the outside. You may think that you know someone just by looking at them but chances are that you don't.
In the middle of the book you also see the theme when everyone thinks that Turtle is the bomber when she isn't. Turtle is actually just covering for the bomber but no one else knows this. "The bomber strikes again.......How I spent my Summer vacation by Turtle Wexler" (Raskin 129). This is an example of the theme because it just shows that Turtle is not who everyone thinks she is.