Control Panel > System > Email Templates
These are the set email templates, these can be edited, click on the one you would like to edit, you can change the subject, header colour, background colour and HTML editor (if you click on the Show HTML Editor, then a textbox will show which can make it a little easier to edit). After any changes hit Save, there is also a preview button that will give you a pop up to allow you to view what the email will look like.
