Sending confirmation emails works, but just text. I want

Sending confirmation emails works, but just text. I want to send them as html. What I did:
- According to the FAQ I added function app_modify_headers( $headers ) e.d. to the bottom of functions.php in the map of my (child) theme.
- In Settings, Add-Ons I activated "Allow HTML emails"

But the emails still come as text. Do I have to schange more things?

Greetings,
Jaap Kolenberg