Function uses_theme 

pub fn uses_theme(book_toml: &str) -> bool

True when book_toml, the text of a book.toml, names a file of the theme directory gruvbox.

assert!(hygiene::book::uses_theme("additional-js = [\"gruvbox/gruvbox.js\"]"));
assert!(!hygiene::book::uses_theme("[book]\ntitle = \"t\"\n"));