brand color shortcodes in .scss don't work with theme: dark

I have:

• searched the issue tracker for similar issues
• installed the latest version of Quarto CLI
• formatted my issue following the Bug Reports guide

Bug description

The colours from palette within _brand.yml can't be used in custom.scss with $brand-color-name when a dark theme is included.

Steps to reproduce

Create a Quarto project with these files and then run quarto preview

````_quarto.yml
project:
  type: website

website:
  title: "Test"
  navbar:
    left:
      - index.qmd

format:
  html:
    theme: 
      light: [brand, cosmo, custom.scss]
      dark: [brand, flatly, custom.scss]
````

````_brand.yml
color:
  palette:
    dark-color: "purple"
````

```` custom.scss
/*-- scss:defaults --*/

/*-- scss:rules --*/
body {
  color: $brand-dark-color;
}
````

````index.qmd
---
title: "Test"
---

This should be purple
````

Actual behavior

You'll get this error

Error: Undefined variable.
      ╷
12847 │   color: $brand-dark-color;

Expected behavior

The brand colours work without the dark mode.

format:
  html:
    theme: [brand, cosmo, custom.scss]

It will also work if custom.scss is not given to dark:

format:
  html:
    theme: 
      light: [brand, cosmo, custom.scss]
      dark: [brand, flatly]

Your environment

• OS: MAC OS 15.4 (24E248)
• RStudio: 2025.05.1 Build 513

Quarto check output

Quarto 1.7.33
[✓] Checking environment information...
      Quarto cache location: /Users/charliejhadley/Library/Caches/quarto
[✓] Checking versions of quarto binary dependencies...
      Pandoc version 3.6.3: OK
      Dart Sass version 1.85.1: OK
      Deno version 1.46.3: OK
      Typst version 0.13.0: OK
[✓] Checking versions of quarto dependencies......OK
[✓] Checking Quarto installation......OK
      Version: 1.7.33
      Path: /Applications/quarto/bin

[✓] Checking tools....................OK
      TinyTeX: (external install)
      Chromium: (not installed)

[✓] Checking LaTeX....................OK
      Using: TinyTex
      Path: /Users/charliejhadley/Library/TinyTeX/bin/universal-darwin
      Version: 2021

[✓] Checking Chrome Headless....................OK
      Using: Chrome found on system
      Path: /Applications/Google Chrome.app/Contents/MacOS/Google Chrome
      Source: MacOS known location

[✓] Checking basic markdown render....OK

[✓] Checking Python 3 installation....OK
      Version: 3.10.14 (Conda)
      Path: /Users/charliejhadley/Library/r-miniconda-arm64/bin/python
      Jupyter: 5.7.2
      Kernels: python3

[✓] Checking Jupyter engine render....OK

[✓] Checking R installation...........OK
      Version: 4.5.1
      Path: /Library/Frameworks/R.framework/Resources
      LibPaths:
        - /Library/Frameworks/R.framework/Versions/4.5-arm64/Resources/library
      knitr: 1.50
      rmarkdown: 2.29

[✓] Checking Knitr engine render......OK

Start by reproducing the issue with _quarto.yml, _brand.yml, custom.scss, and index.qmd, then run quarto preview using the reported light and dark theme configuration. Trace how the brand palette is made available to custom.scss in each theme; done means $brand-dark-color resolves in both themes without the undefined-variable error.