Sphinx provides the _static directory for css, javascript, etc.

Documentation

official docs https://docs.readthedocs.io/en/stable/guides/adding-custom-css.html

CSS && Javascript

${PROJECT}/_static/
  css/*.css
  js/*.js

html_static_path = ['_static']
html_css_files = ['css/custom.css']
html_js_files = ['js/custom.js']