Thanks for installing the WinUI NuGet package!
Don't forget to set XamlControlsResources as your Application resources in App.xaml:
If you have other resources, then we recommend you add those to the XamlControlsResources' MergedDictionaries.
This works with the platform's resource system to allow overrides of the XamlControlsResources resources.
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:controls="using:Microsoft.UI.Xaml.Controls">
See http://aka.ms/winui for more information.