app/templates/layout.html

<!DOCTYPE html>
<html lang="en">
  <head>
    <title>{% block title %}{%- if title %}{{ title }}{% endif -%}{% endblock %}</title>
    <meta charset="utf-8">
    <meta name="viewport" content="width=device-width, initial-scale=1">

    <script src="{{ url_for('static', filename='js/lib/jquery.min.js') }}"></script>
    <script src="{{ url_for('static', filename='js/lib/bootstrap.min.js') }}"></script>
    <script>
      $(document).ready(function() {
      // get current URL path and assign 'active' class
      var pathname = window.location.pathname;
      $('.navbar-nav > li > a[href="'+pathname+'"]').parent().addClass('active');
    })
    </script>

    <link rel="stylesheet" href="{{ url_for('static', filename='css/lib/bootstrap.min.css') }}">
    <link rel="stylesheet" href="{{ url_for('static', filename='css/style.css') }}">
  </head>
  <body>
    <nav class="navbar navbar-expand-lg">
    <a class="navbar-brand mb-0 h1" href="http://re-cite.org">re-cite.org</a>
  <div class="collapse navbar-collapse" id="navbarNav">
    <ul class="navbar-nav">
      <li class="nav-item">
        <a class="nav-link" href="{{ url_for('index') }}">Home</a>
      </li>
      <li class="nav-item">
        <a class="nav-link" href="{{ url_for('about') }}">About Us</a>
      </li>
      <li class="nav-item">
        <a class="nav-link" href="http://github.com/recite/re-cite.org">GitHub</a>
      </li>
      <li class="nav-item">
        <a class="nav-link" href="{{ url_for('contact') }}">Contact Us</a>
      </li>
    </ul>
  </div>
</nav>
    <div class="container main-container">
    {%- with messages = get_flashed_messages(with_categories=true) -%}
      {%- if messages -%}
        {%- for category, message in messages -%}
          {%- if category == 'failed' -%}
            {%- set class = 'danger' -%}
          {%- else -%}
            {%- set class = 'success' -%}
          {%- endif -%}
      <div class="alert alert-{{ class }} alert-dismissible">
        <button type="button" class="close" data-dismiss="alert">&times;</button>
        {{ message|safe }}
      </div>
        {%- endfor -%}
      {%- endif -%}
    {%- endwith -%}
      <div class="mb-5 page-header">
        <br>&nbsp;
        <h4>{% block header %}{%- if header %}{{ header }}{% endif -%}{% endblock %}</h4>
      </div>
    {%- block body %}{% endblock -%}
      <footer class="mt-5 page-footer">
        Unsure about how to use the site? Here's a <a href="{{ url_for('how_to') }}">short video</a> to help you get started.<br><br>
        <div class="text-center">
          © 2018 Copyright: <a href="http://re-cite.org">re-cite.org</a>
        </div>
      </footer>
    </div>
  </body>
</html>