Hi Stergios,
Thank you for contacting Syncfusion Support
In our SfTreeNavigator control, you can apply theme "Dark" and "Light" by defining the color. We have created a simple sample by defining the color brushes for both Light and Dark theme in App.xaml. The defined color gets applied based on the property "RequestedTheme"
Please download the sample from the below link
Sample link:
SfTreeNavigatorWinrtSampleIf your requirement is different from this sample, please provide more information about your requirement.
Regards,
M.Sugapriya