Late to this post but it may help someone else needing help.
*add gift card option to store *
> Home
> Commerce
> Customer Notifications
> Gift Cards (Issued)
> Edit email
You can change the global email style for specific colours for your branding, however I find theres limited options here versus the actual design aspect of Squarespace websites.
To change this gift card option specifically, you can add new text and images to the email.
Hover over the lines for the the (+) content blocks - theres the option for image, text, spacer and line here so you can customise your emails a little more, however it's quite limited.
Good luck!