How to customize the mail notification template?
To adjust the text and subject of the notifications, follow the instructions available here: Changing the text and subject of the notification
Editing the styling of the notifications via UI is not implemented yet, vote for it on Plesk UserVoice. To adjust the styling of the notifications (Logo, CSS, HTML structure, etc.) via configuration files proceed here: Customizing the look of the notifications - Plesk Obsidian Administrator's Guide.