WinRT apps check if the Windows theme is set to dark mode through IUISettings3::GetColorValue().
An option needs to be added to Winecfg to enable dark mode support, which just sets the registry key AppsUseLightTheme to 0 for dark mode and 1 for light mode. It will have to be in a separate merge request. Also, not sure if a dark mode checkbox should be added or if dark mode should be automatically set based on the loaded theme in Wine. Though, for the latter there needs to be a property that returns the mode of the theme. Name alone might not be sufficient.
-- v3: windows.ui/tests: Add IUISettings3::GetColorValue() tests. windows.ui: Implement IUISettings3::GetColorValue(). windows.ui: Add IUISettings3 stub interface. windows.ui: Add stub DLL.