Add timetable.css to substitution.html

aleksis/apps/chronos/templates/chronos/substitution.html

 {% extends 'core/base.html' %}
 
-{% load i18n %}
+{% load i18n static %}
+
+{% block extra_head %}
+  <link rel="stylesheet" href="{% static 'css/chronos/timetable.css' %}">
+{% endblock %}
 
 {% block content %}
   <script type="text/javascript">