Premium subscribers are able to send email newsletters. In New email newsletter templates on 31 May 2023 Manton says:

Today we’re launching a revamped template system for these emails, bringing much more control over what the emails look like. You can edit the template to add a header or footer, or change the HTML tags completely.

You can see what the default email template HTML source looks like here on GitHub. plug-ins can also override this template! So you could have a plug-in that provided a new email design. There’s a help page here with more details about variables you can use in a template. …

You can edit the template under Newsletter → Settings.