something similar was requested here.
https://vborg.vbsupport.ru/showthrea...highlight=html
personally i'd like two boxers in the acp options where you could make the html header and the html footer for your emails and then just add the body on a per-email basis
but you'd also need the user option to select emails in text format of html, so it's not really a simple job but i think my way would be the best way