RTD compatibility.

doc/conf.py

@@ -91,7 +91,12 @@ pygments_style = 'sphinx'
 
 # The theme to use for HTML and HTML Help pages.  See the documentation for
 # a list of builtin themes.
-html_theme = 'nature'
+
+on_rtd = os.environ.get('READTHEDOCS', None) == 'True'
+if on_rtd:
+    html_theme = 'default'
+else:
+    html_theme = 'nature'
 
 # 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