If you make Editor tools for your team or work on Editor UI in some way, the new Editor Foundations Design System is for you! The Unity UX-Foundations team just launched this free design system packed full of helpful guidance, best practices and other resources to help users build better Editor UI.
Please give the EFDS a try and let us know what you think by hitting the “Feedback” button on the EFDS site. And please spread the word as we’d love the EFDS to benefit as many users as possible. Thanks lots!!
I absolutely love this! As a tool dev, this is insanely valuable to me! No more sitting and dissecting the editor with the UI toolkit debugger! 
I’m really looking forward to some sort of icon library. That has always been the hardest part of making editors. I hope it will show us what you actually need to put in the code to actually fetch the icons.
Simply amazing.