Feature: unified light and dark brand

[gordonwoodhull]

Allows specifying light and dark brands in one file, by customizing colors in color or typography with light: and dark:, e.g.

brand:
  color:
    foreground: 
      light: '#222'
      dark: '#eee'
    background:
      light: '#eee'
      dark: '#222'

This introduces two brand schemas, "single" and "unified". Hopefully these names are clear; happy to iterate.

brand.light and brand.dark only take single brand.

brand takes a unified brand (either inline or from a file). We split into light and dark brands and everything from that point is the same as before.

If there are no dark customizations then the unified brand has no dark mode.

Detecting whether dark mode is enabled is now depends on the resolved brand, looking at format.render instead of just format.metadata.

I think that the same two-schema approach will clarify brand.logo for dark logo #12341.

cc @gadenbuie

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