The default renderer uses a handlebars template to render your markdown files and comes with a default theme included in the mdBook binary.

The theme is totally customizable, you can selectively replace every file from the theme by your own by adding a theme directory in your source folder. Create a new file with the name of the file you want to override and now that file will be used instead of the default file.

Here are the files you can override:

• index.hbs is the handlebars template.
• book.css is the style used in the output. If you want to change the design of your book, this is probably the file you want to modify. Sometimes in conjunction with index.hbs when you want to radically change the layout.
• book.js is mostly used to add client side functionality, like hiding / un-hiding the sidebar, changing the theme, ...
• highlight.js is the JavaScript that is used to highlight code snippets, you should not need to modify this.
• highlight.css is the theme used for the code highlighting
• favicon.png the favicon that will be used

Generally, when you want to tweak the theme, you don't need to override all the files. If you only need changes in the stylesheet, there is no point in overriding all the other files. Because custom files take precedence over built-in ones, they will not get updated with new fixes / features.

Note: When you override a file, it is possible that you break some functionality. Therefore I recommend to use the file from the default theme as template and only add / modify what you need. You can copy the default theme into your source directory automatically by using mdbook init --theme just remove the files you don't want to override.