force rtd theme

Since Aug, 22 rtd doesn't force its theme anymore, leaving it as default

docs/conf.py

@@ -42,6 +42,7 @@ release = __version__
 extensions = [
     "sphinx.ext.autodoc",
     "sphinx.ext.napoleon",
+    "sphinx_rtd_theme",
     "sphinxarg.ext",
 ]
 
@@ -66,7 +67,7 @@ exclude_patterns = []
 # The theme to use for HTML and HTML Help pages.  See the documentation for
 # a list of builtin themes.
 #
-html_theme = "default" if on_rtd else "alabaster"
+html_theme = "sphinx_rtd_theme"
 
 # Add any paths that contain custom static files (such as style sheets) here,
 # relative to this directory. They are copied after the builtin static files,