<!DOCTYPE html>
<html>
    <head>
        <title>{% block title '' %} - My Silex Application</title>

        <link href="{{ asset('css/main.css') }}" rel="stylesheet" type="text/css" />

        <script type="text/javascript">
            var _gaq = _gaq || [];
            _gaq.push(['_setAccount', 'UA-XXXXXXXX-X']);
            _gaq.push(['_trackPageview']);

            (function() {
                var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true;
                ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js';
                var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s);
            })();
        </script>
    </head>
    <body>
        {% block content %}{% endblock %}
    </body>
</html>

<!DOCTYPE html>
<html>
    <head>
        <title>{% block title '' %}</title>

        {% block stylesheets %}
            <link href="{{ asset('Semantic-UI-CSS-master/semantic.min.css') }}" rel="stylesheet" type="text/css" />
            <link href="{{ asset('css/main.css') }}" rel="stylesheet" type="text/css" />
        {% endblock %}
    </head>
    <body>
        {% block content %}{% endblock %}
        <script
                src="https://code.jquery.com/jquery-3.1.1.min.js"
                integrity="sha256-hVVnYaiADRTO2PzUGmuLJr8BLUSjGIZsDYGmIJLv2b8="
                crossorigin="anonymous"></script>
        <script src="{{ asset('/Semantic-UI-CSS-master/semantic.min.js') }}"></script>
        {% block javascript %}{% endblock %}
    </body>
</html>

Add Semantic UI to layout template
Add Semantic UI to layout template
