Where is the CiviCRM UI styleguide / reference?
This was covered during CiviCon St. Louis.  I'll try to sum up.


There's no way to write an extension that works with both pre-Shoreditch and post-Shoreditch conventions.
However, it will be a fairly common site in Civi for some time to come to have extensions that implement Shoreditch operating inside of pages that don't.  Mosaico is a good example of this.
Shoreditch is just one implementation of a Civi theme - the future may hold others.  However, for practical purposes I think we can assume there aren't any others coming for a while.


So your options are to either a) ignore all this and do things the old way, or b) implement the Shoreditch theme and make it clear that it's a "dependency" in the docs (until we get for-real dependency management for extensions).