I am assuming you mean all the buttons to make text bold, underline it, etc. Am I right?

If this is the case, you just need to make the body field of the message an html one.

_columns = {
    ...
    'body': fields.html('Body of the email'),
    ...
}

and in the xml, by just putting it in the form view, the default widget has this buttons.

Hope this is what you want!