# Theme

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 next to `src` folder in your project root. 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.
- head.hbs is appended to the HTML `<head>` section.
- header.hbs content is appended on top of every book page.
- css/ contains the CSS files for styling the book.
  - css/chrome.css is for UI elements.
  - css/general.css is the base styles.
  - css/print.css is the style for printer output.
  - css/variables.css contains variables used in other CSS files.
- 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.svg and favicon.png the favicon that will be used. The SVG version is used by newer browsers.

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` and just remove the files you don't want to override.

`mdbook init --theme` will not create every file listed above. Some files, such as `head.hbs`, do not have built-in equivalents. Just create the file if you need it.

If you completely replace all built-in themes, be sure to also set `output.html.preferred-dark-theme` in the config, which defaults to the built-in `navy` theme.