a11y: secondary text (blockquotes, captions, asides) fails WCAG AA color contrast in cosmo

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 body-secondary mixin, which colors blockquotes, figure/table captions, header section numbers, and margin/aside text, derives its color as theme-dim($body-color, 25%) with no contrast check:

https://github.com/quarto-dev/quarto-cli/blob/8577ed1c69b3b9a551f1669c8256abc4a1731266/src/resources/formats/html/bootstrap/_bootstrap-mixins.scss#L1-L7

For theme: cosmo this yields #747a7f on #ffffff = 4.34:1, below the WCAG 2.0 AA (1.4.3) minimum of 4.5:1 — axe-core flags it as color-contrast, impact serious. On quarto.org (a cosmo-derived theme with $body-color: #343a40) the same derivation yields #6d7a86 = 4.39:1, which is how this surfaced in an axe audit of the site.

All five @include body-secondary sites are affected (_bootstrap-rules.scss lines 663, 695, 702, 710, 913), e.g. the blockquote rule:

https://github.com/quarto-dev/quarto-cli/blob/8577ed1c69b3b9a551f1669c8256abc4a1731266/src/resources/formats/html/bootstrap/_bootstrap-rules.scss#L909-L914

Quarto already has contrast-guarding machinery — theme-contrast() / accessibleContrast() with an explicit "AA" (4.5:1) level — used for navbar, footer, sidebar, and button foregrounds:

https://github.com/quarto-dev/quarto-cli/blob/8577ed1c69b3b9a551f1669c8256abc4a1731266/src/resources/formats/html/bootstrap/_bootstrap-functions.scss#L14-L54

Routing the mixin's fallback through it, e.g. theme-contrast(theme-dim($body-color, 25%), $body-bg, "AA"), would keep the dimmed look while guaranteeing conformance for every theme.

Two notes:

• The mixin already honors a $body-secondary SCSS variable when defined — quarto.org uses that as a workaround (quarto-dev/quarto-web#2102). It appears to be undocumented; worth documenting regardless of the fix here.
• Not every built-in theme fails: the default theme's body color dims to #5a6570 = 5.95:1, which passes. The failure depends on where $body-color lands, which is exactly why a contrast guard rather than a per-theme tweak seems right.

Related: the axe-finding family in #14373 and the accessibility epic #8706.

Investigated with AI assistance grounded in a local clone of quarto-cli (per CONTRIBUTING.md, "Using AI tools to investigate").

Steps to reproduce

---
title: repro
format:
  html:
    theme: cosmo
    axe:
      output: document
---

> A blockquote.

Render, open in a browser.

Actual behavior

The blockquote text renders at #747a7f on #ffffff = 4.34:1. axe-core reports:

Serious · WCAG 2.0 AA (1.4.3): Ensure the contrast between foreground and background colors meets WCAG 2 AA minimum contrast ratio thresholds
Elements must meet minimum color contrast ratio thresholds
p

The same color applies to figcaption/caption/.table-caption, .header-section-number, and margin/aside text.

Expected behavior

Text colors derived by Quarto for built-in themes meet the WCAG AA 4.5:1 minimum, e.g. by passing the body-secondary fallback through the existing theme-contrast(..., "AA") machinery.

Your environment

• IDE: Positron 2026.07.0 (build 352)
• OS: macOS 26.5.1 (build 25F80)

Quarto check output

Quarto 1.10.12
[✓] Checking environment information...
      Quarto cache location: /Users/charlottewickham/Library/Caches/quarto
[✓] Checking versions of quarto binary dependencies...
      Pandoc version 3.8.3: OK
      Dart Sass version 1.87.0: OK
      Deno version 2.7.14: OK
      Typst version 0.14.2: OK
[✓] Checking versions of quarto dependencies......OK
[✓] Checking Quarto installation......OK
      Version: 1.10.12
      Path: /Applications/quarto/bin
[✓] Checking tools....................OK
      TinyTeX: v2026.04
      Chrome Headless Shell: 150.0.7871.115
      VeraPDF: 1.28.2
[✓] Checking LaTeX....................OK
      Using: TinyTex
      Path: /Users/charlottewickham/Library/TinyTeX/bin/universal-darwin
      Version: 2026
[✓] Checking Chrome Headless....................OK
      Using: Chrome Headless Shell installed by Quarto
      Path: /Users/charlottewickham/Library/Application Support/quarto/chrome-headless-shell/chrome-headless-shell-mac-arm64/chrome-headless-shell
      Version: 150.0.7871.115
[✓] Checking basic markdown render....OK
[✓] Checking R installation...........OK
      Version: 4.6.0
      Path: /Library/Frameworks/R.framework/Versions/4.6/Resources
      LibPaths:
        - /Users/charlottewickham/Library/R/arm64/4.6/library
        - /Library/Frameworks/R.framework/Versions/4.6/Resources/library
      knitr: 1.51
      rmarkdown: 2.31
[✓] Checking Knitr engine render......OK
[✓] Checking Python 3 installation....OK
      Version: 3.12.2
      Path: /Users/charlottewickham/.pyenv/versions/3.12.2/bin/python3
      Jupyter: 5.9.1
      Kernels: python3
[✓] Checking Jupyter engine render....OK
[✓] Checking Julia installation...

Start with src/resources/formats/html/bootstrap/_bootstrap-mixins.scss and the five body-secondary sites in _bootstrap-rules.scss. Read theme-contrast() and accessibleContrast() in _bootstrap-functions.scss, then reproduce the cosmo example with axe enabled. Done means the affected secondary text styles meet the WCAG AA 4.5:1 contrast threshold across themes.