A new key for notification mails is added to as-config-defaults.yml

# Defines basic styling for notification mails. If you
# override these you must all mandatory elements!
notificationMails:
# Call to actions are often shown as buttons. Their
# appearance can be configured in terms of font color,
# background color and border color.
# Mandatory!
button:
textColor: '#ffffff'
backgroundColor: '#3c73aa'
borderColor: '#356697'
# Notification mails can contain a footer section consisting of
# a text and logo/image. To omit footers at all, you can omit this
# section.
footer:
# Images are referenced via their file name below
# '/opt/open-xchange/templates'. If you don't want any image
# to be included, omit this key.
image: 'ox_logo_claim_blue_small.png'
# The footer text can be customized. If no text shall be
# displayed, omit this key.
text: ''

This key may be overridden per brand in the as-config.yml