I think that design principles are important to bring clarity within the team at a fundamental level. Everything else will build on it. I want you to understand the extreme importance of them. Here is a presentation to let you know more about it.
“They are a tool that helps project teams establish a common language, make day to day decisions, and deliver a better, more consistent user experience. An added benefit of design principles is that the process of defining and agreeing on them helps a project team form a shared understanding of the design goals for a product. This presentation discusses how to create design principles that stick and how to use them to speed up decision making and drive user experience improvements.”