What is the theme of the Art of Racing in the Rain?

One way to think about the theme is that it signifies the contrast between contemplating coming death and the need to focus on the moment when racing, especially when racing in the rain since that is particularly dangerous. Enzo believes this focus is vital to racing because a single memory or speculative thought--like contemplating coming death and transformation in the afterlife--can distract enough to invite an accident and catastrophe. This is an expression of the "be her now" philosophy.