MahApps.Metro is a project that Paul Jenkins started back in 2011 as a simple way to bring a Metro-style user interface into your WPF application. Since then it’s evolved and taken contributions (at last count there were 600+ forks) from various people in the community.
Windows Template Studio use MahApps.Metro to create Metro-Style apps using Windows Presentation Foundation (WPF) framework.
The generated projects includes the MahApps.Metro nuget package, it also adds a new license in the project.
For more info about how to work with MahApps.Metro and WPF see MahApps.Metro GitHub repository.