Thanks for getting back to us, obsite.

For now, you're unable to edit HTML for your invoice email. However, QuickBooks Online gives you the tools to create attractive, professional-looking invoices, estimates, and sales receipts

You can Import a customized invoice so you can design your own invoice including your logo. Make sure to map your import fields correctly to successfully import them to QuickBooks Online. 

Also, editing layout and colors for your invoices can only be done in Custom Form Styles. From there, you'll be able to personalize the appearance and layout of your sales forms.

I've also attached an article for your reference: Email and Customise Sales Form.

Leave a comment below if you need help from us. We're always right here for you.