How to add custom css file to Sphinx?

A simpler way is to add this to your conf.py:

def setup(app):
    app.add_css_file('css/custom.css')  # may also be an URL

Then put the file into the _static/css/ folder.

You should be able to include custom css by extending the default sphinx theme. In your conf.py you would specify where your extension to the theme would be, such as.

# Add any paths that contain templates here, relative to this directory.
templates_path = ['_templates']

Then in _templates you would create a extension to the default theme named 'layout.html' that would include your cssfiles such as.

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

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

See sphinx's documentation on templating for more information.