What is the theme in Lisa, Bright and Dark by John Neufeld?

Mental illness is a recurring idea. Lisa Shilling's behavior seems strange to her school friends at first but as things get worse, they begin to worry. Over time, Betsy, M.N. and even Elizabeth sense that Lisa's problems are deep and complex. When Betsy and M.N. start reading about psychological disturbances, they are led in many directions because they attempt to diagnose Lisa themselves. What the girls eventually come to understand is that mental illness should only be treated by professionals — this is what it took for Lisa Shilling. Lisa's friends are well-meaning but they are certainly amateurs. Lisa's symptoms and behavior deteriorate in the face of her friends' best intentions.