Fix button.

biscuit/apps/chronos/templates/chronos/tt_week.html

@@ -15,7 +15,7 @@
     <ul id="timetable_select_form">
       {{ select_form.as_ul }}
     </ul>
-    <button type="submit" class="btn btn-secondary">
+    <button type="submit" class="btn btn-dark">
       {% blocktrans %}Select{% endblocktrans %}
     </button>
   </form>