The design principals are abstract and lofty. Things like "Enchant Me", "Simplify My Life", and "Make Me Amazing" are pretty open to interpretation, but at least it gives you an idea what the designers were thinking when they created the interface.
The Style section of the site describes things like how to design for different resolutions, what themes would compliment the native UI, their Roboto font, icons, and finally writing style
The writing style is most interesting to me. Here are their main guidelines.
- Keep it brief
- Keep it Simple
- Be Friendly
- Put the most important thing first
- Describe only what's necessary and no more