feature: dark brand

[gordonwoodhull]

Fixes #12159 (in a more general way).

Addresses #11487

Example:

brand:
  dark: dark-brand.yml
  light: light-brand.yml

Collate dark brand in brand sass bundles and use it for creating dark sass bundles.

resolveBrand now returns {light?:, dark?:}, basic downstream changes have been made.¹ May need adjustment.

Enable toggling when brand is multi and includes dark (as previously for theme).

Includes some smoke tests; probably some edge cases need to be tested further.

Footnotes

1. e.g. to still use .light for all the places we are pulling logo. No attempt yet to apply dark logo where needed: that would be a separate effort. ↩

[gordonwoodhull]

Properly collates dark and light bundles, including interaction with themes.

E.g. to use light and dark themes but have dark and light brand.yml override them, use

theme:
  dark: [dark-theme, brand]
  light: [light-theme, brand]

Next steps

• reflect dark brand in lua metadata #12365
  There will be no Pandoc metadata change at least for now, because Typst is the only producer and consumer of brand metadata for Pandoc partials, and there can only be one brand
• project config can specify light and dark brand files #12367
• dashboard dark brand - to my surprise, it mostly works already. There are some glitches with renderings: [light, dark] due to In dashboard, plt.show() show intermediate matplotlib output #11150
• test reveal (which does not support switching but we should test that it takes first brand)
• dark logo #12341 — delayed to 1.8
• playwright tests for dark brand #12358

Will file PRs for these separately.