Dark mode does not set `data-bs-theme`, so Bootstrap colour-mode styles never apply

导航栏已经在 nav-before-body.ejs 中自己的元素上设置了 data-bs-theme="dark"，所以先从那里看这个模式。然后找到把 quarto-dark 添加到 body 的深色模式切换代码，在同一位置为 html 设置 data-bs-theme，切回时使用 light。使用 issue 中的 dark.scss 和 index.qmd 复现，切换模式，并检查 .btn-close 是否应用了 invert 滤镜。当属性跟随切换变化，并且深色背景上的关闭按钮可见时，即完成。

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

In dark mode, Quarto marks the page with body.quarto-dark and swaps the stylesheet bundle, but it never sets data-bs-theme="dark".
Bootstrap 5.3 keys its colour-mode rules on that attribute, so Bootstrap components keep their light-mode look on a dark page.

The clearest case is .btn-close.
By default its icon is a black SVG, and Bootstrap only lightens it under [data-bs-theme="dark"].
On a dark surface such as a modal, an offcanvas or a toast, the close button is close to invisible.

This is also an accessibility problem in Quarto's own output.
In a dashboard with light and dark themes and axe: output: document, the accessibility report panel stays white in dark mode, while its close button takes the dark theme's white icon.
The close button cannot be seen.
Setting data-bs-theme="dark" by hand makes it show again.

The navbar already sets data-bs-theme="dark" on its own element in nav-before-body.ejs, but nothing sets it for the page.

Steps to reproduce

dark.scss:

/*-- scss:defaults --*/
$body-bg: #1e1e2e;
$body-color: #f0f0f0;

index.qmd:

---
title: "Dark mode and Bootstrap colour modes"
format:
  html:
    theme:
      light: cosmo
      dark: [cosmo, dark.scss]
---

```{=html}
<div class="modal-content p-3" style="max-width:320px">
  <div class="d-flex justify-content-between align-items-center">
    <strong>Modal header</strong>
    <button type="button" class="btn-close" aria-label="Close"></button>
  </div>
</div>
<div class="alert alert-warning alert-dismissible mt-3" role="alert" style="max-width:420px">
  A dismissible alert.
  <button type="button" class="btn-close" aria-label="Close"></button>
</div>
```

Render, open the page, and switch to dark mode with the toggle.

Actual behavior

After the switch:

• body has the class quarto-dark.
• document.documentElement.getAttribute("data-bs-theme") is null.
• .btn-close keeps the black icon (fill='%23000') with no filter, so it is barely visible on the dark page.
• .alert-warning keeps its light tint, rgb(255, 227, 209).

Expected behavior

Dark mode sets data-bs-theme="dark" on <html>, and light mode sets it back to light, so Bootstrap's colour-mode rules apply.
The close button then turns light on dark surfaces.
Setting the attribute by hand in the browser after the switch gives .btn-close the filter invert(1) grayscale(1) brightness(2), which comes from the [data-bs-theme=dark] .btn-close rule already in the compiled dark bundle.

The missing attribute may also explain other dark-mode reports, such as #13173 (navbar dropdown hover) and #13195 (tooltip contrast).

Your environment

• IDE: none, rendered from the command line.
• OS: macOS 27.0.

Quarto check output

quarto check

Quarto 99.9.9

[✓] Checking environment information...
      Quarto cache location: /Users/mcanouil/Library/Caches/quarto
      Project: none found (single-file mode)

[✓] 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: 99.9.9
      commit: b86f98d382940bef6c65f37ea0bdfba844f19b6d
      Path: /Users/mcanouil/Projects/quarto-dev/quarto-cli/package/dist/bin

[✓] Checking tools....................OK
      TinyTeX: v2026.07
      Chrome Headless Shell: (not installed)
      VeraPDF: (not installed)

[✓] Checking LaTeX....................OK
      Using: TinyTex
      Path: /Users/mcanouil/Library/TinyTeX/bin/universal-darwin
      Version: 2026

[✓] Checking Chrome Headless....................OK
      Using: Chrome from QUARTO_CHROMIUM
      Path: /Applications/Brave Browser.app/Contents/MacOS/Brave Browser

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

ℹ R version 4.6.1 (2026-06-24)
! Config '~/.Rprofile' was loaded!

[✓] Checking R installation...........OK
      Version: 4.6.1
      Path: /Library/Frameworks/R.framework/Versions/4.6/Resources
      LibPaths:
        - /Users/mcanouil/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: 3.13.11
      Path: /Users/mcanouil/Projects/q2-tests/.venv/bin/python3
      Jupyter: (None)

      Jupyter is not available in this Python installation.
      Install with python3 -m pip install jupyter

[✓] Checking Julia installation...