This! Windows, Linux and macOS are OSes with very different aesthetics and concepts. Trying to emulate these (sometimes subtle) differences with a webpage won't work. And it will totally destroy the illusion if an OS update comes with a new theme.

And let's not forget that you're basically running a completely separate browser that can't re-use any memory from the other 5 separate browsers (=Electron apps) you need to run for work all day.