lefrenchmathproject/_includes/header.html

<header class="site-header">
    <style>
        header {
            position: fixed;
            top: 0;
            width: 100%;
            z-index: 1000; /* Ensure it stays above other content */
            background-color: #fff; /* or any background you prefer */
            box-shadow: 0 2px 5px rgba(0, 0, 0, 0.1); /* Optional: adds a shadow to make it stand out */
            }

        body {
        padding-top: 60px; /* Adjust according to your header's height */
        }

        body.dark-mode header {
        background-color: #333;
        color: white;
        }
    </style>

    <div class="wrapper">
      {%- assign default_paths = site.pages | map: "path" -%}
      {%- assign page_paths = site.header_pages | default: default_paths -%}
      {%- assign titles_size = site.pages | map: 'title' | join: '' | size -%}
      <a class="site-title" rel="author" href="{{ "/" | relative_url }}">{{ site.title | escape }}</a>
  
      {%- if titles_size > 0 -%}
        <nav class="site-nav">
          <input type="checkbox" id="nav-trigger" class="nav-trigger" />
          <label for="nav-trigger">
            <span class="menu-icon">
              <svg viewBox="0 0 18 15" width="18px" height="15px">
                <path d="M18,1.484c0,0.82-0.665,1.484-1.484,1.484H1.484C0.665,2.969,0,2.304,0,1.484l0,0C0,0.665,0.665,0,1.484,0 h15.032C17.335,0,18,0.665,18,1.484L18,1.484z M18,7.516C18,8.335,17.335,9,16.516,9H1.484C0.665,9,0,8.335,0,7.516l0,0 c0-0.82,0.665-1.484,1.484-1.484h15.032C17.335,6.031,18,6.696,18,7.516L18,7.516z M18,13.516C18,14.335,17.335,15,16.516,15H1.484 C0.665,15,0,14.335,0,13.516l0,0c0-0.82,0.665-1.483,1.484-1.483h15.032C17.335,12.031,18,12.695,18,13.516L18,13.516z"/>
              </svg>
            </span>
          </label>
  
          <div class="trigger">
            {%- for path in page_paths -%}
              {%- assign my_page = site.pages | where: "path", path | first -%}
              {%- if my_page.title -%}
              <a class="page-link" href="{{ my_page.url | relative_url }}">{{ my_page.title | escape }}</a>
              {%- endif -%}
            {%- endfor -%}
          </div>
        </nav>
      {%- endif -%}
    </div>
  </header>