feat(admin): dark mode

[antfu]

Should we have dark mode for admin? If so, should we also controls the theme of preview to match with the admin?

[atinux]

Of course we need to support dark mode of the admin like we have for Maintainers. I don't know yet if we need to sync between admin and the website since you may not want to use the editor in light mode if you use to code with dark theme for example.