Custom Email Header and Footer Template in Magento 2

You can override these templates in your custom theme and edit the html as per your requirement.

app/design/frontend/Vendor/themename/Magento_Email/email/header.html
app/design/frontend/Vendor/themename/Magento_Email/email/footer.html

You can override your core header and footer template into your theme folder,

vendor/magento/module-email/view/frontend/email/header.html

To your theme,

app/design/frontend/{Vendorname}/{thememane}/Magento_Email/email/header.html

Same Process for Footer.html file.

app/design/frontend/{Vendorname}/{thememane}/Magento_Email/email/footer.html

Run

php magento setup:upgrade
php magento setup:static-content:deploy

Clear cache.

This setting available in:

Content -> Configuration -> Default Store View -> Transactional Emails

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