Addendum to:
One should never say we aren't trying. It doesn't work. It can only style very simple pages. Furthermore, we'd have to allow unsafe-inline scripts as it modifies the page styling in a rather dirty way. This is not really something we want to do (note that the test page doesn't have Content Security rules at all).
Compare normal
to "dark mode"
I'm pretty sure you'd find this "dark mode" insufficient.