Enable a default dark theme to support CSS prefers-color-scheme media query

Last year, Discourse added support for automatic color theme switching based on device preferences: Dark Mode Automatic Theme Switcher - theme - Discourse Meta.

Would you enable this for the NixOS instance? You might have to set some instance-wide theme defaults. It should work regardless of whether visitors are signed in, I think. See https://discourse.codinghorror.com for a live example.

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