Use "navy" as the default theme as "dark" is not a built-in theme.

QuineDot · QuineDot · commit 54d0a3d180a4 · 2023-07-17T18:38:19.000-07:00
diff --git a/book.toml b/book.toml
@@ -11,6 +11,6 @@ edition = "2021"
 
 [output.html]
 git-repository-url = "https://github.com/QuineDot/rust-learning"
-default-theme = "dark"
+default-theme = "navy"
 site-url = "/rust-learning/"
 fold.enable = true
