brand.yml not being picked up by quarto preview in extension

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

Bug description

I'm creating a quarto extension for a custom Typst output format and I want to include brand.yml in this project. According to the docs, we have to add the brand.yml specs to the _extension.yml file like so:

contributes:
  metadata:
    project:
      brand: brand.yml

This works when using quarto render but not when using quarto preview. For some reason, preview does not pick up on the brand style changes.

Steps to reproduce

The template.qmd file

---
title: My custom extension
format:
  quarto-workshop-typst: default
---

## Section A


Lorem ipsum dolor sit amet, consectetur adipiscing elit. Phasellus vel commodo nibh. Nunc eleifend purus ac tincidunt condimentum. Donec eget sodales nibh. Sed lacinia viverra rutrum. Nullam rutrum suscipit blandit. Nulla facilisi. Cras id orci ipsum. Ut id orci neque. Donec purus augue, luctus non tristique a, ultricies vitae leo. Etiam eu imperdiet ante. Sed convallis turpis at turpis interdum commodo. Fusce fringilla faucibus massa eget iaculis. Quisque lacinia varius augue in gravida. Mauris consectetur, dui sit amet viverra egestas, eros nibh fringilla eros, ut sagittis nisl nunc a purus.

The _extension.yml file

title: Quarto Workshop Extension
author: Albert Rapp
version: 1.0.0
quarto-required: ">=1.10.0"
contributes:
  metadata:
    project:
      brand: _brand.yml
  formats:
    typst:
      template-partials:
        - typst-template.typ
        - typst-show.typ

The _brand.yml file:

meta:
  name: brand.yml
  link: https://posit-dev.github.io/brand-yml

color:
  palette:
    black: "#1A1A1A"
    white: "#FFFFFF"
    orange: "#FF6F20"
    pink: "#FF3D7F"
  foreground: black
  background: white
  primary: orange
  danger: pink

typography:
  fonts:
    - family: Inter
      source: google
    - family: Rubik
      source: google
    - family: IBM Plex Mono
      source: google

  base: Inter
  headings:
    family: Rubik
    weight: 400
  monospace: IBM Plex Mono

Actual behavior

Preview shows the unbranded version

Expected behavior

Preview shows the branded version

Your environment

Positron Version: 2026.08.1 build 2
Code - OSS Version: 1.124.0
Commit: 48dc4cee686939f35fe4a714d34375f67fd62082
Date: 2026-08-09T13:35:16-06:00
Electron: 42.2.0
Chromium: 148.0.7778.97
Node.js: 24.15.0
V8: 14.8.178.14-electron.0
OS: Linux x64 6.8.0-136-generic

Quarto check output

Quarto 1.10.18
[✓] Checking environment information...
      Quarto cache location: /home/albert/.cache/quarto
[✓] Checking versions of quarto binary dependencies...
      Pandoc version 3.10.0: OK
      Dart Sass version 1.101.0: OK
      Deno version 2.7.14: OK
      Typst version 0.15.1: OK
[✓] Checking versions of quarto dependencies......OK
[✓] Checking Quarto installation......OK
      Version: 1.10.18
      Path: /opt/quarto/bin

[✓] Checking tools....................OK
      TinyTeX: (not installed)
      Chrome Headless Shell: (not installed)
      VeraPDF: (not installed)

[✓] Checking LaTeX....................OK
      Using: Installation From Path
      Path: /usr/bin
      Version: 2021

[✓] Checking Chrome Headless....................OK
      Using: Chrome found on system
      Path: /usr/bin/google-chrome
      Source: PATH

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

[✓] Checking R installation...........OK
      Version: 4.4.3
      Path: /usr/lib/R
      LibPaths:
        - /home/albert/R/x86_64-pc-linux-gnu-library/4.4
        - /usr/local/lib/R/site-library
        - /usr/lib/R/site-library
        - /usr/lib/R/library
      knitr: 1.51
      rmarkdown: 2.31

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

[✓] Checking Python 3 installation....OK
      Version: 3.10.12
      Path: /usr/bin/python3
      Jupyter: 5.5.0
      Kernels: ark, python3

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

[✓] Checking Julia installation...