Where is Django base.html file? Add additional static style sheet

I would like copy & use the admin/base.html file in my templates/accounts folder. So I can hopefully include one additional custom style sheet in the head. I have used the:

python -c "import django; print(django.__path__)"

previously to locate & copy the user admin forms. When I follow the path generated from the line above I now get a path to .py files rather than .html files.

Does anyone have a copy of the base.html file? Or can suggest the cleanest way to include one additional style sheet?

This is what I am trying to avoid if possible.

base_site.html

{% extends "admin/base.html" %}
{% block title %}{{ title }} | {{ site_title|default:_('MY SITE') }}{% endblock %}

<!--CAN THIS BE ADDED TO THE HEAD OF base.html-->
{% block body_head %}
<link href="{% static 'css/custom.css' %}" rel="stylesheet">
{% endblock body_head %}

{% block branding %}
<h1 id="site-name"><a href="{% url 'admin:index' %}">MY SITE</a></h1>
{% endblock %}
{% block nav-global %}{% endblock %}