Theme and symbolism are two very different things.
The theme of a piece of literature is what it is all about, the "central and dominating idea." Since this book is an autobiography, there's a pretty good chance that the theme is the story of the life of Augusten Burroughs. To state it better, though, Magill's Literary Annual states of this book: "Running with Scissors is an outstanding example of the abuses that can and do go on in homes in this society unbeknownst even to the neighbors."
Symbolism is the use of images to represent an idea or emotion or concept. It may also include giving special, symbolic, meaning to an object or idea. One example of symbolism in this book is the title: Running with Scissors symbolizes the neglect and danger Burroughs faced in his childhood.