Make theme toggle the ability to follow system scheme

[gouku]

I really like how the new MDN handle themes:

[slorber]

We don't provide this out of the box and don't really plan to, but you can swizzle Navbar/ColorModeToggle and add your own UX

Note we don't support setColorMode(undefined) to reset to system scheme, but that could be useful to support 👍

[pranabdas]

I think you can use following configuration to query and apply system color mode instead of a hardcoded default.

colorMode: {
          respectPrefersColorScheme: true,
},

https://docusaurus.io/docs/api/themes/configuration#respectPrefersColorScheme