Do you have any internal line-of-business apps or add-ins that was designed to look like Office?
In this case, they may be using a library called Fabric JS. Please note that.
Over the past few years, the team behind Office UI Fabric has been working hard on Office UI Fabric React. It is a robust framework for building experiences that fit seamlessly into a broad range of Microsoft products. It includes many full-featured, production-ready UI components, utilities for accessibility, styling, iconography, toolkits, resources for designers, and more.
Going forward, it is recommended that developers use Fabric React and Fabric Core to build experiences that align with the Office user experience. While Microsoft is no longer actively developing new features or fixing bugs, all existing versions of Fabric JS will remain available on Microsoft’s CDN and solutions built with Fabric JS will continue to function normally.
If you have, or are building, applications that leverage Office UI Fabric JS, you can continue using it; however, Microsoft will not fix any bugs and offers no guarantee that components will stay current with latest Office user experience.
If you would like to ensure your applications fit seamlessly into a broad range of Microsoft products, instead of using Fabric JS, we recommend using Fabric React or Fabric Core.