Fix deprecation error

docs/source/conf.py

@@ -278,5 +278,5 @@ def setup(app):
         'enable_auto_doc_ref': False,
     }, True)
     app.add_transform(AutoStructify)
-    app.add_stylesheet('custom.css')  # may also be an URL
+    app.add_css_file('custom.css')  # may also be an URL
     build_api_docs(app.outdir)