add non_exhaustive attribute on Theme, Chapter

src/book/book.rs

@@ -152,6 +152,7 @@ impl From<Chapter> for BookItem {
 /// The representation of a "chapter", usually mapping to a single file on
 /// disk however it may contain multiple sub-chapters.
 #[derive(Debug, Clone, Default, PartialEq, Serialize, Deserialize)]
+#[non_exhaustive]
 pub struct Chapter {
     /// The chapter's name.
     pub name: String,

src/theme/mod.rs

@@ -45,6 +45,7 @@ pub static FONT_AWESOME_OTF: &[u8] = include_bytes!("FontAwesome/fonts/FontAweso
 /// You should only ever use the static variables directly if you want to
 /// override the user's theme with the defaults.
 #[derive(Debug, PartialEq)]
+#[non_exhaustive]
 pub struct Theme {
     pub index: Vec<u8>,
     pub head: Vec<u8>,