https://docs.google.com/document/d/1xYr6lFyV8QAPrW7-_qJ4zXXQ7Id32PFeRN7RyC9cHuQ/...
In the FAQ the second point:
"Q: May I enhance / change / alter graphics?
A: If you want to edit the graphics, this licence allows you to do so, although every edit or enhancement will be handled as „derivative work“ and may not be redistributed/sublicensed (3.1C, 5)"