How do I create custom email branding for a catalog or subcatalog using the Storefront Theme Editor?
As a Catalog admin, if the Storefront Theme Editor has been enabled for your account, you can create custom email branding for your catalog and subcatalogs using HTML.
If custom email branding is not created for a subcatalog, it inherits the parent catalog email branding.
Note: When the parent catalog has the Modern Storefront enabled, all subcatalogs automatically inherit this setting and its branding elements such as logos, colors, and favicons, which cannot be customized at the subcatalog level. However, if the parent catalog uses the Legacy Storefront, subcatalogs can enable the modern Storefront independently and manage their own branding settings.
Open Admin

Click the User Name drop-down menu [1]. Then, click the Admin link [2].