Use 'sphinx_rtd_theme' as theme

doc/api/conf.py

@@ -27,7 +27,7 @@
 
 # Add any Sphinx extension module names here, as strings. They can be extensions
 # coming with Sphinx (named 'sphinx.ext.*') or your custom ones.
-extensions = ['sphinx.ext.autodoc', 'sphinx.ext.viewcode']
+extensions = ['sphinx.ext.autodoc', 'sphinx.ext.viewcode', 'sphinx_rtd_theme']
 autoclass_content = 'both'
 autodoc_default_flags = ['members', 'undoc-members', 'inherited-members']
 autodoc_member_order = 'bysource'
@@ -100,7 +100,7 @@
 
 # The theme to use for HTML and HTML Help pages.  See the documentation for
 # a list of builtin themes.
-#html_theme = 'classic'
+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