What is the theme in The Fourth Turning: An American Prophecy by William Strauss?

Cyclical time is a theme in the book. The Fourth Turning is a book of prophecy, so it naturally concerns the future. As such, it also appeals to events in the past and present to predict the future. Time is center stage. However, the authors have an argument to make: the West has lost a crucial understanding of time in the last several centuries. For the last few centuries, Westerns have understood time as 'linear' - as always either progressing or regressing. The future builds on the past in a constant, often steady fashion.