I have 4 machines 2 Dev with VS / Telerik and 2 Test both are clean Win 10 machines. When I run the project below in Dev it works perfectly but in Test the theme get's all screwed up and goes mostly transparent.
I have tried this with couple of other themes and it works fine and I have tried it with the Tab Control rather than the TabbedWindow and that also works. It looks like a combination of Fluent and TabbedWindow breaks.
To recreate, make a simple 1 page App, add the TabbedWindow and some content, then set the Fluent Theme in the code behind.
You have to test this on a "clean" machine, without VS or Telerik installed for it to break.