Fix deprecations in current theme

[marble]

/ALL/userhome/.local/share/virtualenvs/venv-y0waPz_e/local/lib/python2.7/
site-packages/sphinx/builders/html.py:138: 
RemovedInSphinx20Warning: builder.css_files is deprecated. 
Please use app.add_stylesheet() instead.
    ret += other

[marble]

Obsolete