With the option 'Enable Email Approval Response' checked, you can approve to text email by replying 'Yes','No', 'APPROVE', 'REJECT',etc. Is possible to reply your approval or rejection on an email that has an html email template?
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Yes you can do that by creating a rich letterheaded e-mail. You can find the process in detail in this SFSE link. Add a logo to the approval alert template
Yes of course you can do it with custom email Template.
The only thing you need to take care of is email template should contain the appropriate information with the link to Approve page
Go throuth Enable Email Approve Response this link
Hope this information will help you out to resolve your query.