docs/source/_templates/layout.html

{# layout.html #}
{# Import the theme's layout. #}
{% extends "!layout.html" %}

{% set css_files = css_files + ['_static/style.css'] %}

{% block footer %}
{{ super() }}

<script async src="https://www.googletagmanager.com/gtag/js?id=UA-45299905-2"></script>
<script>
  window.dataLayer = window.dataLayer || [];
  function gtag(){dataLayer.push(arguments);}
  gtag('js', new Date());

  gtag('config', 'UA-45299905-2');
</script>

{% endblock %}