Dark Mode should be auto-enabled if OS prefers

[systemPreferences's isDarkMode](https://www.electronjs.org/docs/api/system-preferences#systempreferencesisdarkmode-macos-windows-deprecated) or the newer [nativeTheme.shouldUseDarkColors](https://www.electronjs.org/docs/api/native-theme#nativethemeshouldusedarkcolors-readonly) should be used to automatically enable the editor's dark mode by default.

P.S.
I'd still keep the ability for users to switch via the menu as they do today. Especially for Linux, where I don't think the "dark/light mode" preference really exists/works.