fix(brand): Set $theme: "brand" when brand is included

[gadenbuie]

Fixes #11471

Sets $theme: "brand" !default in the brand Sass layers to achieve this desired behavior:

I think the best we can do is to use the name that the configuration deemed to be "the most important", eg theme: [brand, cosmo] gets $theme: "cosmo" and theme: [cosmo, brand] gets $theme: "brand".

Test with the example in #11471 and use this _quarto.yml:

project:
  type: website

website:
  title: "Brand navbar-bg default"
  navbar:
    left:
      - index.qmd

format:
  html:
    theme: [brand, cosmo]
    # theme: [cosmo, brand]
    # theme: brand

[cscheid]

Lol #11474

[gadenbuie]

Something about great minds...! I'll close mine, yours has the needed if (brand) :)