There is a number of documentation resources from Apple that may be of help:
And a number of Stack Overflow questions as well:
https://stackoverflow.com/questions/25379525/how-to-detect-dark-mode-in-yosemite-to-change-the-status-bar-menu-icon
https://stackoverflow.com/questions/51672124/how-can-it-be-detected-dark-mode-on-macos-10-14
https://stackoverflow.com/questions/39048894/how-to-detect-switch-between-macos-default-dark-mode-using-swift-3
Often you'll need to convert objective-c stack overflow / tutorials / apple documentation to C# and this documentation will help:
https://developer.xamarin.com/guides/mac/application_fundamentals/patterns/
https://developer.xamarin.com/guides/mac/application_fundamentals/mac-apis/
Answers
There is a number of documentation resources from Apple that may be of help:
https://developer.apple.com/documentation/appkit/supporting_dark_mode_in_your_interface?language=objc
And a number of Stack Overflow questions as well:
https://stackoverflow.com/questions/25379525/how-to-detect-dark-mode-in-yosemite-to-change-the-status-bar-menu-icon
https://stackoverflow.com/questions/51672124/how-can-it-be-detected-dark-mode-on-macos-10-14
https://stackoverflow.com/questions/39048894/how-to-detect-switch-between-macos-default-dark-mode-using-swift-3
Often you'll need to convert objective-c stack overflow / tutorials / apple documentation to C# and this documentation will help:
https://developer.xamarin.com/guides/mac/application_fundamentals/patterns/
https://developer.xamarin.com/guides/mac/application_fundamentals/mac-apis/