I think that give the users the ability to shoe their notebook without installing anything would be great, so +1 to add the css or maybe an option in the config to automatically load the notebook css...
Thinking in more things about this... surely we can provide more things to make the notebook users happy... I have to revisit my workflow (a lot of time wo publishing) to give more feedback...
The other thing, but that's is bigger than Nikola would be a system on the cloud with the notebook as UI to write your post and easy deploy of the website, all in the cloud without installing anything... just ready to login and publish... I am planning some experiment on this path in the future.