This is really a follow-up to my earlier post about tips with SharePoint publishing customisations. Composed looks have been a part of a couple of projects recently. In the first, a solution for on-premise, we used code in a feature receiver to add a number of items to the Composed Looks list. In the second, for Office 365, a bit of research offered an alternative approach with no code.
What are Composed Looks
A composed look is a collection of master page, colour scheme file, font scheme file and background image. There is a site list called Composed Looks that holds them, and they are shown in the Change the Look page as the thumbnail options you can choose to apply branding in one hit.