The document needs to be in the folder:
Mac : {user}:Library:Application Support:Adobe:Adobe Illustrator {version}:{lang}:New Document Profiles
Win : {drive}\Documents and Settings\{user}\Application Data\Adobe\Adobe Illustratorn {version} Settings\{lang}\New Document Profiles
Those folders are hidden by default. Please refer to your system's documentation on how to show them. So you might need to save the file somewhere else first and then move it to that folder.
Ah, Monika! That was exactly what I was looking for:
The folder mentioned by you (which is btw. not invisible on my Mac system) is containing the whole shebang of "defined default documents" used as basic for any new document to be opened (e.g. print.ai, web.ai, mobile.ai) - and they can beedited.
So, instead of clumsily creating a "new document style" I can simple open those default documents, kill all unneeded colours, brushes etc. and now every time I create a new document (e.g. "print"), they're no longer there.
I have tested and can approve this procedure.
So thanks a lot for your kind response - although not exactly the road you wanted me to take, your hint helped me to solve my problem!