If you want to edit the HTML 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.
Note: content created using the Handlebars override is not reusable across channels. If you need to use Handlebars for a portion of your email, use template blocks and combine them with other block types to allow your non-handlebars code content to be re-used in other 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.