The Stationery in Mail has been removed in Mojave (Apple didn't explain the reason). Custom stationery files are still available in the user's library folder ~/Library/Containers/com.apple.mail/Data/Library/Application Support/Mail/Stationery/Apple/Contents/Resources/Custom/Contents/Resources/.
If you still have a backup copy of High Sierra or lower it seems it's possible to revert back this feature:
mail.app - How to get stationery back in Apple Mail on macOS Mojave? - Ask Different
On-line there're various services available to build “rich” e-mails (e.g. newsletters). I name just two: BEE (https://beefree.io/) and TOPOL (Topol.io - beautiful e-mail editor and builder). Usually free and paid versions are available.
Another option, if you're familiar with coding, is to build the e-mail using free e-mail frameworks available on-line. Again, I name just two: MJML (https://mjml.io/) and ZURB Foundation (https://foundation.zurb.com/emails/email-templates.html).
Regardless the service/tool used, if you still want to rely on Mail to send the generated e-mail code you can preview it in Safari and then compose the e-mail by sharing the page invoking File -> Share -> Email This Page.