If you’re sure you’ve got a DNS entry for the Pihole FQDN pointing at Traefik, open the dev panel in your browser (F12), switch it to the Network tab, and visit the pihole URL.
See if you get anything back and especially take note of the HTTP status codes.
I’m not sure about color support without HTML or add-ons, but Obsidian is a good markdown editor with a lot of functionality and extensibility.
It’s not open source but it runs on everything.