Dark mode for composer, conversation and signature webviews

This MR adds dark mode support to the composer, conversation & signature web views, using :root { color-scheme: light dark }

Plain text emails render nicely.

Most HTML emails I've received render nicely. Many specify background: black !important; color: white !important on body, and these remain unchanged.

See commit messages for more info.

Closes #588

Thanks 🙂