Como posso adicionar um arquivo css personalizado? A seguinte configuração não funciona para mim:

# conf.py
html_static_path = ['_static']
html_theme = 'default'
html_theme_options = {
  'cssfiles': ['_static/style.css']
}

Resultado:

C:\temp\test-docs\docs>make html
Running Sphinx v1.2.2
loading pickled environment... not yet created
building [html]: targets for 2 source files that are out of date
updating environment: 2 added, 0 changed, 0 removed
reading sources... [ 50%] help
reading sources... [100%] index

looking for now-outdated files... none found
pickling environment... done
checking consistency... done
preparing documents...
Theme error:
unsupported theme option 'cssfiles' given