FEATURE: Enable auto dark mode on new instances (PR #14208)

Sets the “Dark” theme as the default dark mode color scheme while seeding data (which only runs on new instances.

This also adds some logic to the wizard to unset the relevant site setting if the default color scheme is dark.


    dark_scheme_id = ColorScheme.where(base_scheme_id: "Dark").pluck_first(:id)
    if dark_scheme_id.present?
      SiteSetting.default_dark_mode_color_scheme_id = dark_scheme_id

Perhaps we can move this to a before block?

          expect { updater.update }.to change { SiteSetting.default_dark_mode_color_scheme_id }.to(-1)

This might work.

Some minor suggestions but look good to me.

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