Fix dark highlighting themes missing base text color

[jdonaldson]

Summary

Fixes #14099

Dark syntax highlighting themes like zenburn and espresso don't set a base text color when paired with a light theme. Unhighlighted code tokens inherit the light theme's near-black color, making them invisible against the dark background.

Root cause

Two issues:

1. $code-block-color never emitted for paired themes (format-html-scss.ts): The text-color extraction from the theme JSON was gated by !themeDescriptor.isAdaptive. When users specify separate light/dark themes (highlight-style: {light: kate, dark: zenburn}), isAdaptiveTheme() returns true because the config object has both light and dark keys — even though neither kate nor zenburn is actually an adaptive theme. So $code-block-color was never set as an SCSS variable.

2. No span-level color rule (_bootstrap-rules.scss): Even with $code-block-color set, it was only applied to container elements (div.sourceCode, div.sourceCode pre.sourceCode). The light theme's syntax highlighting CSS generates code.sourceCode > span { color: #1f1c1b; } which has higher specificity than container inheritance.

Fix

1. format-html-scss.ts: Move the text-color → $code-block-color extraction outside the !isAdaptive guard. Adaptive themes handle their own background-color, but text-color still needs to propagate for Bootstrap rules to use.

2. _bootstrap-rules.scss: Add span-level selectors (pre > code.sourceCode > span, code.sourceCode > span) alongside the existing container-level $code-block-color rules. This follows the same pattern as the existing background-color handling.

Affected themes

7 built-in themes missing "Normal" in text-styles: espresso, haddock, monochrome, none, pygments, tango, zenburn. Also covers user-created custom themes with the same structure.

Verified

Built quarto from source and tested:

• {light: kate, dark: zenburn} → dark CSS now includes code.sourceCode > span { color: #ccc; }
• {light: kate, dark: espresso} → dark CSS now includes code.sourceCode > span { color: #bdae9d; }
• Light mode unchanged (kate's rules still present, no regressions)

cc @mcanouil — this follows the _bootstrap-rules.scss pattern you suggested, plus fixes an upstream gating issue where $code-block-color was never emitted for paired (non-adaptive) themes.

[posit-snyk-bot]

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

Status	Scanner	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.