[Don-Yin]

Description

Fixes #15176.

quarto-listing.scss opened its variables with /*-- scss:variables --*/, which is not a layer boundary, so they stayed in the functions layer and were set before the theme's !default values. I changed it to /*-- scss:defaults --*/.

The same thing overrode the grays and border color of the Bootswatch themes. I rendered a website with each built-in theme before and after, and its :root colors now match a standalone document with the same theme (slate, for example, no longer gets light #dee2e6 borders). The border radius stays at the listing's 0.25rem unless the theme sets one.

The new smoke-all test fails on main and passes with the change, and the smoke/site tests pass.

Disclosure: I used Pi with Claude Code.

Checklist

I have (if applicable):

• referenced the GitHub issue this PR closes
• updated the appropriate changelog in the PR
• ensured the present test suite passes
• added new tests
• created a separate documentation PR in Quarto's website repo and linked it to this PR

AI-assisted PR

• AI tool used: Pi with Claude Code
• Codebase grounding: local clone
• Human review: I have reviewed, tested, and verified the AI-generated content before submitting.

Note: autonomous AI agents submitting PRs without human oversight are not permitted — see the Code of Conduct.

[posit-snyk-bot]

✅ Snyk checks have passed. No issues have been found so far.

Status	Scan Engine	Critical	High	Medium	Low	Total (0)
✅	Open Source Security	0	0	0	0	0 issues
✅	Licenses	0	0	0	0	0 issues

💻 Catch issues earlier using the plugins for VS Code, JetBrains IDEs, Visual Studio, and Eclipse.