In LiveWhale 1.6+, you can customize the HTML template that LiveWhale uses for sending bulletins and other notifications.
The default template is located on your server at /livewhale/core/modules/messages/includes/email_template.html. (Note that's the back-end /livewhale/ folder, not the one in your front-end directory).
To create your own custom template, make a copy of that file and upload it to /livewhale/client/modules/messages/includes/email_template.html. Then you can make any changes you like to that version.
LiveWhale 1.6+ will check for the existence of a custom template in /livewhale/client/ and, if none exists, fall back to the default template in /livewhale/core/.