What's the best approach for global theme resources in WPF Class Library when App.xaml is not available?

What is the recommended approach for globally registering theme resources in a WPF class library ?

Are there official patterns or alternatives to App.xaml that can provide similar global resource resolution capabilities in this scenario?

We've also created a discussion in the official WPF repository: https://github.com/dotnet/wpf/discussions/11075