You can find the Mailing settings under the third Communication icon in the left navigation menu, in the Mailing section.
Start by clicking the New Email Set button.
In the email editor, you can configure the following parameters that will apply to all emails sent in the set (e.g., Sent, Ready for Pickup, Delivered, etc.):
Color scheme (links, background, text, and more).
Button color settings.
After saving the email set, you need to activate the new Mailing settings with the Activate button.