Top 20 NuGet theming Packages

Build native UIs for iOS, Android, and Windows Phone from a single, shared C# codebase
Light Theme for Xamarin Forms applications
Dark Theme for Xamarin Forms applications
Elysium is library that implements Metro style for Windows Presentation Foundation applications.
Elysium Extra is a library that implements Metro style for Windows Presentation Foundation (WPF) applications. This Project is a very large add-on project built on top of the Elysium SDK.
A quick way to theme an MVC Razor website
Take a look at https://cssvariableshim.azurewebsites.net and try it out there.
Ljund Controls contains Windows 8 themeing for WPF and helpers for animation of controls such as fading in/out, resizing, sliding and more. In addition to these core features, a TextEntryDialog is provided, and a helper for navigation between pages in a WPF Frame.
Enables implementing themes for ASP.NET MVC
A Theming system for Umbraco (min Umbraco version 7.12)
Enables implementing themes for ASP.NET MVC
A Theming system for Umbraco (min Umbraco version 8)