fix: derive bone colors from the text color instead of the OS scheme (BON-13)

[hunterbecton]

Fixes BON-13: on a dark-mode OS over a page that keeps a light canvas, bones resolved to white-at-12% and vanished. The prefers-color-scheme media query assumed the page follows the scheme; pages auto.css targets often don't.

--bone-base and --bone-highlight now default to the inherited text color at 12% and 6% opacity, and the media query is gone. Bones contrast with whatever background the page's own text is readable on: light pages get dark bars, dark pages and dark cards get light bars, and pages that never set a color get exactly the old rgba(0, 0, 0, 0.12) — existing visual baselines pass unchanged.

The subtle part: bones hide their content through the color property, so the issue's literal color-mix(… currentColor …) reads transparent at every bone and paints nothing (validated in the comparison prototype, which also ruled out fixed translucent grays — they vanish on gray backgrounds). Content is instead hidden by zeroing only the alpha, rgb(from currentColor r g b / 0), so the channels survive into the derivation. The measured overlay's no-stylesheet fallbacks derive the same way.

Costs worth knowing:

• Requires relative color syntax (Baseline 2024: Chrome 119, Safari 18, Firefox 128) — hence a minor bump.
• Per-element tints don't survive: a blue link's bone renders in the parent's color, since the hiding declaration wins the cascade on the element itself.
• Text that relies on its background for contrast (white hero text over an image) still needs a local --bone-base override; now documented in the styling page.

New browser tests pin the derivation for data-bone text and block bones, auto.css leaves and blocks, the alt-text channel survival, and the shadow overlay fallback, plus one back-compat pin for default-color pages. Verified end to end with Playwright colorScheme: "dark" over an undeclared white page — dark, visible bars.

Part of BON-13.

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Summary by CodeRabbit

• New Features

  • Skeleton and overlay colors now automatically adapt to the surrounding text color.
  • Improved contrast across light and dark themes without requiring separate theme-specific defaults.
  • Content hidden beneath skeletons remains visually concealed while preserving adaptive coloring.
  • Overlay fallback styles now match adaptive color behavior.

• Documentation

  • Updated styling guidance with adaptive color behavior, theme support, and manual override options.

• Bug Fixes

  • Improved handling of white-on-white content and measured overlays.

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• packages/bones/src/element/overlay.ts
• packages/bones/src/css/auto.css
• packages/bones/src/css/bones.css

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📝 Walkthrough

Walkthrough

Bone colors now derive from inherited text color instead of fixed light and dark values. Alpha-based hiding replaces transparent. Overlay fallbacks use the same derivation. Browser tests, documentation, sandboxes, and release notes cover the change.

Changes

Adaptive bone colors

Layer / File(s)	Summary
CurrentColor-based bone rendering packages/bones/src/css/bones.css, packages/bones/src/css/auto.css, packages/bones/src/element/overlay.ts	Bone defaults, shimmer colors, reduced-motion styles, and automatic skeleton colors derive from currentColor. Hidden content uses zero alpha.
Adaptive color browser coverage packages/bones/tests/browser/adaptive-colors.test.ts, packages/bones/tests/browser/overlay-fallback.test.ts, packages/bones/tests/browser/expect-color.ts	Browser tests validate inherited colors, hidden content, fallback colors, overlay rendering, and color parsing assertions.
Theme documentation and release notes apps/docs/content/docs/styling.mdx, packages/bones/sandbox/*.html, .changeset/adaptive-bone-colors.md	Documentation and sandbox comments describe text-color-relative rendering, theme behavior, overrides, and relative color syntax support.

Estimated code review effort: 2 (Simple) | ~10 minutes

Merge Risk: ⚪ Minimal · up to fdfbc

The PR updates bone colors to follow inherited text color and adds coverage for the new behavior. No actionable merge-blocking risk remains beyond normal checks and review.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 33.33% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 3 functions across 4 files. (2 skipped: 2…	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the main change: deriving bone colors from text color instead of the OS color scheme. It also includes the relevant issue identifier.
Description check	✅ Passed	The description provides a detailed What/Why explanation and a Testing section with test coverage, verification details, compatibility notes, and known limitations.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

Full details: Docstring Coverage

Explanation

Docstring coverage is 33.33% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 3 functions across 4 files. (2 skipped: 2 unsupported.)

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Inline comments:
In `@packages/bones/src/css/bones.css`:
- Around line 8-9: Replace the `currentColor` keyword with the configured
lowercase `currentcolor` spelling in `packages/bones/src/css/bones.css` lines
8-9, 15, and 74, and in `packages/bones/src/css/auto.css` lines 47 and 147; no
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Configuration used: defaults

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Reviewing files that changed from the base of the PR and between 19a9dcb and 7b05416.

📒 Files selected for processing (10)

• .changeset/adaptive-bone-colors.md
• apps/docs/content/docs/styling.mdx
• packages/bones/sandbox/auto.html
• packages/bones/sandbox/boundary.html
• packages/bones/src/css/auto.css
• packages/bones/src/css/bones.css
• packages/bones/src/element/overlay.ts
• packages/bones/tests/browser/adaptive-colors.test.ts
• packages/bones/tests/browser/expect-color.ts
• packages/bones/tests/browser/overlay-fallback.test.ts

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