Remove broken styles reference:

The error I saw was:
> You have set your config to `serve_locally=True` but A local version of https://codepen.io/briandennis/pen/zRbYpB.css is not available.
If you added this file with `app.scripts.append_script` or `app.css.append_css`, use `external_scripts` or `external_stylesheets` instead.
See https://dash.plotly.com/external-resources

Author: susodapop

demo.py

@@ -34,8 +34,7 @@
 
 app = dash.Dash("")
 
-app.css.append_css(
-    {"external_url": "https://codepen.io/briandennis/pen/zRbYpB.css"})
+# TODO: styles are required
 
 app.config.suppress_callback_exceptions = True
 app.scripts.config.serve_locally = True