Apply dev-mode style in Django admin area

templates/admin/base_site.html

+{% extends "admin/base.html" %}
+{# Django-provided default template: #}
+
+{% block title %}{{ title }} | {{ site_title|default:_('Django site admin') }}{% endblock %}
+
+{% block branding %}
+<h1 id="site-name"><a href="{% url 'admin:index' %}">{{ site_header|default:_('Django administration') }}</a></h1>
+{% endblock %}
+
+{% block nav-global %}{% endblock %}
+
+{# Local additions: #}
+{% load static %}
+{% block extrastyle %}
+    {% if debug %}
+        <link rel="stylesheet" href="{% static 'common/css/dev.css' %}">
+    {% endif %}
+{% endblock %}