Clean up the theme files now that header.hbs is a thing.

No more having to track index.hbs between updates. \o/

There was an extra book.css in src/theme as far as I could tell.

Author: Havvy

book.toml

@@ -1,2 +1,8 @@
+[book]
+
 title = "The Rust Reference"
 author = "The Rust Project Developers"
+
+[output.html]
+
+additional-css = ["theme/reference.css"]