The Tufte theme suggests modifying the config.json file. @rnv helpfully provided instructions.

It's a little fiddly, but here's what you do:

  1. go to the "Design" area.
  2. Scroll down to "Custom Theme."
  3. Click on "Edit Custom Themes." — "Tufte theme" should be listed there.
  4. Click on it.
  5. look for config.json and click on it.
  6. scroll to the very bottom of the code
  7. You'll see "subtitle" and "mailto" — edit them.
  8. click "Update Template"