you all may know how to customize the body of a workflow notification (sent by a Send Notification or by any task action) but, what if you want to change the default header/footer (with the orange horizontal line and "Workflow Notification" text)?
You can do these changes at three different level: farm / site collection / web and there you can also change the default text for approval and the approval no longer required notification:
In Central Administration, going into Nintex Administration you can find the Message templates link:
For Web, you have to go to Site settings and select Message templates
from the page that opens, you can then switch to site collection settings:
In each of the three pages, you have four boxes where you can customize the template, if needed you can use common references, basic item properties and inline functions: