Actually, the site owner has known about this open-source WPF control library for a long time, but just couldn't appreciate its style. However, they still starred the project. Every time they browse the repository, they see the star count growing and many users praising it, so today they recommend this WPF control library to everyone.
1. How good is it? Just look at the repository screenshot:
Screenshot of the MahApps.Metro repository on GitHub

Currently supports .NET Framework (4.5+) and .NET Core (3.0 and 3.1).
Open source under the MIT license:
MIT is a permissive license similar to BSD; the author only wishes to retain copyright without any other restrictions. In other words, you must include the original license notice in your distribution, whether you distribute it in binary or source form.
- You may use, copy, and modify the software.
- You may use the software for free or sell it.
- The only restriction is that it must be accompanied by the MIT license.
2. A great open-source project’s quality can be seen from the number of contributors:

3. Projects based on this control library
- Azuser by @Inzanit Azure SQL Server User Management
- Carnac by @Code52 the Magnificent Keyboard Utility
- Certify The Web by @webprofusion The GUI to manage free certificates from Let's Encrypt
- Chocolatey GUI Chocolatey GUI is a user interface for Chocolatey (a Machine Package Manager for Windows)
- CodeTrack by Nico Van Goethem CodeTrack is a versatile profiler with some extra tricks up its sleeve.
- Espera by @flagbug
- FolderSecurityViewer Easy to use NTFS permissions reporter to get all effective security owners of your data
- Hearthstone-Deck-Tracker by @Epix37 HDT is an automatic deck tracker and manager for Hearthstone
- Markdown Edit by @mike-ward A full-featured Markdown editor for Windows with an emphasis on content and keyboard shortcuts
- Markdown Monster by @RickStrahl An extensible Markdown Editor, Viewer and Weblog Publisher for Windows
- MarkPad by @Code52 a visual Markdown editor
- Modern UI for WPF (MUI) Inspired by MahApps.Metro
- myEcho iOS Dictation for Windows
- NETworkManager by @BornToBeRoot A powerful tool for managing networks and troubleshoot network problems!
- Papercut by @Jaben Simple Desktop SMTP Server / Email Receiver
- SimpleMP by @punker76 Simple Music Player - SimpleMP - Keeps it simple and plays your music
- Solutionizer by @thoemmi Creating ad-hoc solutions for Visual Studio
- WinReform by @AKruimink A simple tool to help resize and relocate stubborn windows.
- Xamarin Inspector Visualize and debug your live app
- Xamarin Workbooks Workbooks is an interactive programming environment that’s perfect for experimentation, learning, and documentation: an educational tool for learning the myriad of .NET platforms, APIs, and libraries. Source
4. Control previews







Finally, an animated image

5. Conclusion
For future WPF projects or the site owner's own open-source projects, they have decided to try incorporating this control library. With so many people using it, it must be good.
- Repository: MahApps.Metro