Update your Global Email Styles:
Customer notifications are automated emails that go out from your site — set them up even if you don't have a shop, since forms and other interactions can trigger them
Start with Global Email Styles — sets the branding for all notification emails at once so you don't have to edit each one individually
What you can customize:
Logo (insert and set height)
Color theme
Font (limited selection compared to your website)
Text colors
Button style
Footer text (e.g. "Thank you for visiting my website")
Social links
Layout options
Reply-to email — set this in Business Settings so when someone replies to a notification, it comes to you directly rather than a generic Squarespace address
Business Settings — add your business name and at least city/state so it appears correctly on outgoing emails
Send a test email to yourself to check how it looks before it goes out to customers
⭐ Core advice: at minimum set up Global Email Styles and your reply-to email — everything else will follow from the global settings