What is the theme in Stone Butch Blues by Leslie Feinberg?

Gender is definitely a theme in this book. Stone Butch Blues is a story of the flexibility of gender, as opposed to sex. We are born as one of the two sexes, male or female (except for the very few who are intersex, both with both male and female genitalia). Gender, however - whether we express masculinity or femininity or alternate between the two - is not necessarily linked to biological sex.