add custom 404 page

app/templates/common/404.html

@@ -0,0 +1,8 @@
+{% extends 'base.html' %}
+{% block title %}not found{% endblock %}
+{% block content %}
+  <div class="darkbg settings-container">
+  	<h1 class="thread-title">404 Not Found</h1>
+  	<p>The requested URL does not exist.</p>
+  </div>
+{% endblock %}