Dark mode for composer, conversation and signature webviews

Open mooff requested to merge mooff/geary:mooff/dark-mode into main

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 🙂

Edited by mooff

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