The general mailer layout (specifically, the footer that appears on emails), kudos mailer, and user invite increase mailer all have HTML in the locale file. It should be banished back to the view files.
As an admin, grant invitations to a user account whose email address you have access to, and ensure that the email looks normal. (Note: This cannot be done by going to https://test.archiveofourown.org/users/USERNAME/invitations –- doing that doesn’t send an email. You have to use the “Give invite codes to current users” option on https://test.archiveofourown.org/admin/invitations, or you have to update the site settings to allow users to request invitations, make a request as a user, and then grant that request as an admin.)
Give yourself kudos (either while logged out or logged in with another account) and ensure that the kudos email looks right.
Please make sure to check the footer in one of those emails.