It's possible to edit the HTML of a Courier Email notification directly using the Handlebars template override.
Override the Notification Designer With Handlebars
If you want to edit the HTML / Handlebars code of an Email Notification directly, you can switch from the Notification Designer to the Handlebars editor.
This is a useful way to import existing HTML templates into Courier.
Content created using the Handlebars override is not reusable across channels.
Edits made in the Handlebars editor will not apply to other Email notifications. To define the look and feel of all your emails at once, customize your default brand template.