_output.yml

 bookdown::gitbook:
    css: [css/book.css, 'https://use.fontawesome.com/releases/v5.0.13/css/all.css']
    font-import: https://fonts.googleapis.com/css?family=Roboto|Open+Sans 
    font-family: 'Roboto' 
    number_sections: false
    split_by: rmd
    config:
      edit : null
      download: null
      search: yes
      sharing:
        facebook: no
        twitter: no
        google: no
        weibo: no
        instapper: no
        vk: no
        all: ['twitter', 'facebook', 'google', 'weibo', 'instapaper']
      toc:
        depth: 2
        collapse: section
        scroll_highlight: yes
        before: |
          <li class='before'><a href="./">STARS</a></li>
        after: |
         <li class='after'">
            <a href="https://m-clark.github.io/">
               <img src="img/mc_logo.png" style="width:25%; padding:0px 0; display:block; margin: 0 auto;" alt="MC logo">
            </a>
         </li>
         <li class='after'">
            <div style='text-align:center'>
               <a href="https://github.com/m-clark/">
                  <i class="fab fa-github fa-2x" aria-hidden="true"></i>
               </a>
            </div>
         </li>
         <li class='after'">
            <div style='text-align:center'>
               <a rel="license" href="http://creativecommons.org/licenses/by-sa/4.0/">
                  <i class="fab fa-creative-commons fa-lg"></i> 
                  <i class="fab fa-creative-commons-by fa-lg"></i> 
                  <i class="fab fa-creative-commons-sa fa-lg"></i>
               </a>
            </div>
         </li>
# bookdown::pdf_book:
#   includes:
#     in_header: preamble.tex
#   latex_engine: xelatex
#   citation_package: natbib
# bookdown::epub_book: default