not able to add custom template files in gitbook themes-open source projects GitbookIO/gitbook

I am trying to create a custom gitbook theme and in that I also want to change the layout so that the book I create using the theme have the layout that I want. I copied the default templates dir in my assests dir of the custom theme and then modified the layout.html and header.html files as I wanted. Then to include the modified template files, I added the following attribute to the index.js file

module.exports = {
    book: {
        assets: "./assests",
        templates: {
            "layout":"templates/layout.html",
            "header":"templates/includes/book/header.html",
        },
......
......

However with this configuration, the generated book is not picking the template file changes. However I do see the css/js changes that I had done.