configure theme as documented at https://sphinx-rtd-theme.readthedocs…

….io/en/stable/installing.html

conf.py

@@ -40,7 +40,8 @@
 # extensions coming with Sphinx (named 'sphinx.ext.*') or your custom
 # ones.
 extensions = [
-    'sphinxcontrib.spelling'
+    'sphinxcontrib.spelling',
+    'sphinx_rtd_theme',
 ]
 
 # Spelling configuration
@@ -122,11 +123,7 @@
 
 # -- Options for HTML output ----------------------------------------------
 
-on_rtd = os.environ.get('READTHEDOCS', None) == 'True'
-if not on_rtd:  # only import and set the theme if we're building docs locally
-    import sphinx_rtd_theme
-    html_theme = 'sphinx_rtd_theme'
-    html_theme_path = [sphinx_rtd_theme.get_html_theme_path()]
+html_theme = 'sphinx_rtd_theme'
 
 # Theme options are theme-specific and customize the look and feel of a theme
 # further.  For a list of options available for each theme, see the