The theme used by the App Builder isn't actually available as a theme style with Universal Theme. So you would need to use a Universal Theme style to provide a similar look&feel in an extension app. Thanks for raising this question - it's certainly something for us to take into consideration.

I wouldn't say that anything like this is planned - there would be a big need for quality assurance and such right.

Our 24.1 API Documentation will be refreshed tomorrow, and will include the APIs Christian's discussed. So please check back tomorrow - our Documentation team are actively working on the updates :-)

Well semantically you can use it for whatever you want, out-of-office or anything else. Name is debatable of course but ‘vacation rule’ is easy to understand and common in WF land