The UI Fabric site is all about sharing the design standards Microsoft follows for their products.
You’ll find fonts, icons, button styles, wireframes and code here.
The item I use most frequently is the THEME GENERATOR.
This tool allows you to specify primary and secondary colors, and it will generate a SharePoint theme palette for you. It shows you how that palette will look on buttons, options, input fields, etc. It produces code, JSON or PowerShell language that you can copy for use on your site.