The Human Interface Guidelines (HIG) provide design resources and best practices for implementing high-quality and consistent UI that delivers easy-to-use and efficient workflows in Unity.
"I tried the new Editor UI for a while and it felt really lightweight and fluid to use."
"It feels natural to me. It makes me want to play around with it."
"Designers and artists will feel more at home here,"
"Having a more up to date interface is nice. This feels more modern."