We maintain a collection of Calypso design templates for Sketch that can be used for designing interfaces for WordPress.com and our native apps. The Sketch document includes form fields, buttons, sample typography, notices, dialogs, dropdowns, sample gravatars/blavatars, and more. This document is a work in progress and some components may be missing our outdated.
A collection of live design patterns can be found in Calypso Docs (requires a WordPress.com account).
Next up: Imagery
WordPress.com Brand Guide 