@toddgrotenhuis I was briefly stumped by this, too. It's a little fiddly, but here's what you do:
- go to the "Design" area.
- Scroll down to "Custom Theme."
- Click on "Edit Custom Themes." — "Tufte theme" should be listed there.
- Click on it.
- look for config.json and click on it.
- scroll to the very bottom of the code
- You'll see "subtitle" and "mailto" — edit them.
- click "Update Template"