Theming

A theme system keeps component roles stable while modes remap values. Components reference semantic tokens such as --color-bg, --color-text, and --color-focus; measure every mode before claiming accessibility. Dark is not inverted light; authored high contrast is not forced colors.

The decision order

1. Semantic tokens first. Define roles (--color-bg, --color-surface-1/2 (-3 if elevation needs it), --color-text, --color-text-muted, --color-border, --color-primary, --color-primary-hover, --color-primary-ink, --color-accent, --color-accent-ink, --color-success, --color-error, --color-warning, --color-info, --color-focus — canonical contract names). Components use these only; palette comes from color-system.
2. Choose the product baseline. Start with the default supported mode; set structure and role relationships there.
3. Tune every additional mapping, never invert it. For dark, use a lighter-per-level surface ramp, comfortable text contrast, and retuned accents. See references/surfaces-and-elevation.md and color-and-contrast.md.
4. Separate authored contrast from forced colors. prefers-contrast: more may select a higher-contrast mapping; forced-colors is system-controlled, so honor its colors.
5. Resolve preferences deliberately. Use prefers-color-scheme, persist selectable themes, and set color-scheme so native controls follow.
6. Verify each theme on its actual surfaces.

Vary mappings, preserve contract

Starter values show role mapping, not a dark recipe. Derive mappings from roles, ambient conditions, brand, and platform constraints; keep role names, forced-colors respect, preference precedence, and measured pairs intact. Missing roles require named recovery; never invent component colors.

Non-negotiables

• One semantic-token source; components never hard-code colors. Re-theming remaps tokens only.
• Choose light surfaces from context. Pure or softened endpoints can both be valid; test contrast, glare, reading comfort, and preferences.
• Dark is tuned, not inverted: near-black is reliable, but pure black can suit OLED, projection, or a stark system. Distinguish elevation with surfaces/borders, tune text below glare-inducing white where appropriate, and remeasure accents.
• Ship a default and resolution policy. A supported user choice persists and outranks OS preference; a single-theme product needs no toggle.
• color-scheme declared per theme (native controls/scrollbars follow).
• Respect forced-colors/high-contrast — don't override system colors; use forced-color-adjust deliberately and keep focus visible.
• Contrast re-verified per theme (body ≥4.5, large/UI ≥3) against that theme's surfaces — a color that passes in light often fails in dark, and vice-versa.

Resolution and recovery

Separate role names, theme values, and selection. Document precedence: system forced-colors; supported user choice; OS preference; application default. For a control, define storage and fallback.

Forced-colors is an environment, not a palette: keep system colors/native controls authoritative and preserve warnings, errors, selections, and focus with text, icons, state, or shape. If a role is absent, record a failed recovery row and stop; reusing an accent changes its meaning.

Apply the theme early to minimize wrong-theme flash. State pre-paint behavior, unavailable saved choices, and native-control color-scheme. No-flash needs a cold-load observation naming theme, artifact, and timing.

Quick-start

Use assets/theme-starter.css; preserve roles, tuned mappings, forced-colors cooperation, and early saved-preference resolution.

Early saved-theme resolution: when a persistent control exists, apply the validated override before render. In a CSP-compatible implementation, this is commonly a small nonced or hashed <head> script rather than a deferred bundle:

<script>/* inline, in <head>, before CSS paint */
  try {
    const t = localStorage.getItem("theme");
    if (t === "dark" || t === "light") document.documentElement.dataset.theme = t;
  } catch {
    /* Storage is unavailable; CSS keeps the OS/application default authoritative. */
  }
</script>

Browser chrome, selection, and custom scrollbars are optional polish, not completion gates.

light-dark() shorthand

When color-scheme drives the switch (no persisted override), light-dark() avoids media-query duplication: background: light-dark(oklch(96% .02 78), oklch(18% .024 64)). Broad browser support (2024+); pair with [data-theme] so explicit choice and forced-colors override. It is not the sole mechanism with a manual toggle.

Reference files

• references/surfaces-and-elevation.md — dark ramp, elevation, borders, shadow limits.
• references/color-and-contrast.md — dark accents, state colors, contrast, OKLCH.
• references/light-and-contrast.md — light mappings, relationships, accents, rendered checks.
• references/high-contrast.md — prefers-contrast + forced-colors; override limits.
• references/decision-records.md — meta-patterns + ADR rules.

Ledger

Close with five-field ledger. Start Reason with the check ID; name mode/environment in Evidence.

How to deliver

Report roles, mappings, resolution, wrong-theme-flash behavior, and per-theme contrast, with scoped-environment and forced-colors proof when applicable. Hand source ramps, type, technical a11y, and cognitive concerns to adjacent skills. Provenance is credited and independent.

Contract (generated)

Canonical detail: contract.json.

• Route: An interface needs coherent light, dark, high-contrast, or forced-colors mappings (+1 in contract.json); avoid: The request is to invent the source palette rather than map semantic roles (+1 in contract.json)
• Exclude: Do not invert a light palette and call it dark mode (+1 in contract.json)
• Stop / handoff: Pause when semantic roles are missing and theme values would be raw-color copies (+1 in contract.json); receives [color-system, design-system-interview, improve-existing-website, tasteroll] -> sends [a11y-pass, cognitive-a11y, responsive-layout, component-states]
• Output: Semantic mappings for every supported theme, plus forced-colors behavior, preference policy, and measured pairs
• Evidence: table_with_evidence with status, reason, remediation, evidence, provenance.