flask_app/templates/layout.html

<!DOCTYPE html>
<html>
    <head>
        <title>{% block title %}{% endblock %}</title>
        <script src="https://code.jquery.com/jquery-3.3.1.slim.min.js" integrity="sha384-q8i/X+965DzO0rT7abK41JStQIAqVgRVzpbzo5smXKp4YfRvH+8abtTE1Pi6jizo" crossorigin="anonymous"></script>
        <!-- <link rel="stylesheet" href="https://use.fontawesome.com/releases/v5.2.0/css/all.css" integrity="sha384-hWVjflwFxL6sNzntih27bfxkr27PmbbK/iSvJ+a4+0owXq79v+lsFkW54bOGbiDQ" crossorigin="anonymous"> -->
        <!-- <link rel="stylesheet" href="https://pro.fontawesome.com/releases/v5.8.1/css/all.css" integrity="sha384-Bx4pytHkyTDy3aJKjGkGoHPt3tvv6zlwwjc3iqN7ktaiEMLDPqLSZYts2OjKcBx1" crossorigin="anonymous"> -->
        <link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.1.1/css/bootstrap.min.css" integrity="sha384-WskhaSGFgHYWDcbwN70/dfYBj47jz9qbsMId/iRN3ewGhXQFZCSftd1LZCfmhktB" crossorigin="anonymous">
        <link rel="stylesheet" href="{{ url_for('static', filename='css/all.css') }}">
        <link rel="stylesheet" href="{{ url_for('static', filename='styles.css') }}">
        <script src="https://ajax.googleapis.com/ajax/libs/jquery/3.2.1/jquery.min.js"></script>
        <script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.14.3/umd/popper.min.js" integrity="sha384-ZMP7rVo3mIykV+2+9J3UJ46jBk0WLaUAdn689aCwoqbBJiSnjAK/l8WvCWPIPm49" crossorigin="anonymous"></script>
        <script src="https://stackpath.bootstrapcdn.com/bootstrap/4.1.1/js/bootstrap.min.js" integrity="sha384-smHYKdLADwkXOn1EmN1qk/HfnUcbVRZyYmZ4qpPea6sjB/pTJ0euyQp0Mk8ck+5T" crossorigin="anonymous"></script>
    </head>
    <body>
        <div class="container" style="margin-top: 40px;">
            {% if error %}
                <div class="alert alert-danger" role="alert">
                    {{ error|safe }}
                </div>
            {% endif %}

            {% if alert %}
                <div class="alert alert-primary" role="alert">
                    {{ alert|safe }}
                </div>
            {% endif %}
            {% block body %}
            {% endblock %}
        </div>
    </body>
</html>