A living style guide is a reference for a team of designers and developers to understand how an application will look and feel. It is used to convey design language, define design rationale, and explain how to use markup. A living style guide creates shared understanding among all team members and is a place for the whole team to reference how something should look and feel.
Living style guides should change and adapt to new needs and challenges that arise during the life of a project. Nothing should be written in stone, and they should consistently be refined and revised.