- ***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.
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.