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bvandeusen 1dcce4e304 docs(m7-362): theme toggle design spec

Web SPA dark/light/system theme. Token strategy: dual :root +
[data-theme="light"] blocks in tokens.generated.css; Tailwind
unchanged. Light palette is parchment/aged-paper. Adds
non-flipping --fs-on-action for action button labels. FOUC
prevented via inline app.html head script.

2026-05-03 13:31:18 -04:00

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M7 #362 — Web SPA Theme Toggle (Dark / Light / System)

Status: Design Milestone: M7 (v1.0 web-first sweep) Issue: Fable #362 Scope: Web SPA only. Flutter mobile mirror is out of scope (separate task in Tier 1 catch-up).

Goal

Let a user choose Dark, Light, or follow-system theme for the Minstrel web SPA. Persist the choice across sessions. Apply it before first paint to avoid FOUC.

Out of scope

Per-page or per-section theme overrides.
Multi-color brand palettes beyond the dark + light pair.
Time-of-day automatic switching independent of the OS preference.
Cross-tab live sync (settings change in tab A doesn't propagate to tab B). Documented as a known limitation.
Flutter client mirror.
Animated transitions between modes (instant flip).

Architecture

Token strategy

Existing system: design tokens defined in web/src/lib/styles/tokens.json, generated to web/src/lib/styles/tokens.generated.css by web/scripts/tokens-to-css.js, and consumed by Tailwind via var(--fs-*) references in web/tailwind.config.js.

Theme is implemented by emitting two blocks of custom-property values in the generated CSS:

:root {
  /* Dark palette (default) + flat tokens used in both modes */
  --fs-obsidian: #0E1013;
  ...
  --fs-moss: #5A7563;
  --fs-on-action: #E8E2D2;
}

[data-theme="light"] {
  /* Light overrides only — flat tokens are NOT repeated */
  --fs-obsidian: #F8F5EE;
  --fs-iron: #ECE6D5;
  ...
}

Tailwind config is not changed. The var(--fs-*) indirection means every utility class (bg-fs-iron, text-fs-parchment, etc.) automatically swaps when the [data-theme="light"] selector matches.

The active theme is selected by setting data-theme="light" (or absent / "dark") on <html>.

tokens.json schema change

Current shape (flat):

{
  "colors": {
    "obsidian": "#0E1013",
    "iron": "#14171A",
    ...
  }
}

New shape:

{
  "colors": {
    "dark": {
      "obsidian": "#0E1013",
      "iron": "#14171A",
      "slate": "#1C1F23",
      "pewter": "#2C313A",
      "parchment": "#E8E2D2",
      "vellum": "#C9C2AE",
      "ash": "#8A8474"
    },
    "light": {
      "obsidian": "#F8F5EE",
      "iron": "#ECE6D5",
      "slate": "#DCD3BD",
      "pewter": "#B8AE94",
      "parchment": "#14171A",
      "vellum": "#2C313A",
      "ash": "#5C6068"
    },
    "flat": {
      "moss": "#5A7563",
      "bronze": "#8C6A3F",
      "oxblood": "#7A2E2E",
      "warning": "#C99A4A",
      "error": "#A04848",
      "info": "#5A7A8A",
      "accent": "#4A6B5C",
      "on-action": "#E8E2D2"
    }
  }
}

flat tokens are emitted once in :root only. They do not flip.

Generator change

web/scripts/tokens-to-css.js is updated to:

Walk colors.dark + colors.flat, emit a :root { ... } block with --fs-<name>: <value>; lines.
Walk colors.light, emit a [data-theme="light"] { ... } block.
Preserve existing non-color sections of tokens.json (typography, spacing, etc.) — those continue to emit into :root only.

`--fs-on-action` token

A new flat token, value #E8E2D2 (frozen at the dark-mode parchment value). Used as the label color on action buttons whose backgrounds (--fs-moss, --fs-bronze, --fs-oxblood) do not flip. Without this, action button text would invert in light mode and lose contrast against the mid-tone earth backgrounds.

Tailwind utility: text-fs-on-action.

Theme store

New file: web/src/lib/stores/theme.svelte.ts.

Exports a small rune-state module:

type ThemePreference = 'dark' | 'light' | 'system';
type ResolvedTheme = 'dark' | 'light';

export const theme: { value: ThemePreference };       // user preference
export const resolvedTheme: { value: ResolvedTheme }; // what's actually applied
export function setTheme(pref: ThemePreference): void;

Implementation:

On module load, read localStorage['minstrel:theme'] (default: 'system').
If 'system', resolve via matchMedia('(prefers-color-scheme: light)') and subscribe to change events to keep resolvedTheme in sync.
setTheme(pref) writes localStorage, updates theme.value, recomputes resolvedTheme.value, and sets document.documentElement.dataset.theme = resolvedTheme.value.

FOUC prevention

web/src/app.html gains an inline script in <head>, before any CSS link:

<script>
  (function () {
    try {
      var t = localStorage.getItem('minstrel:theme') || 'system';
      var resolved = t === 'system'
        ? (matchMedia('(prefers-color-scheme: light)').matches ? 'light' : 'dark')
        : t;
      document.documentElement.setAttribute('data-theme', resolved);
    } catch (e) { /* localStorage unavailable — accept default dark */ }
  })();
</script>

This runs synchronously before paint, so the correct theme is applied on first render. SSR/prerender paths see no theme attribute server-side; the script applies it client-side before paint.

Settings UI

web/src/routes/settings/+page.svelte gains an "Appearance" card at the top:

┌─ Appearance ─────────────────────┐
│ Theme:                           │
│   ( ) Dark    (•) Light    ( ) System │
│                                  │
│   System follows your device's   │
│   light/dark preference.         │
└──────────────────────────────────┘

Implemented as three radio inputs (or a segmented control component if one already exists in the project — implementer to check). Selection writes through setTheme().

Light palette spec

Full token map with values:

Token	Dark (current)	Light (new)	Role
`--fs-obsidian`	`#0E1013`	`#F8F5EE`	page bg
`--fs-iron`	`#14171A`	`#ECE6D5`	card/surface bg
`--fs-slate`	`#1C1F23`	`#DCD3BD`	hover surface
`--fs-pewter`	`#2C313A`	`#B8AE94`	borders
`--fs-parchment`	`#E8E2D2`	`#14171A`	primary text
`--fs-vellum`	`#C9C2AE`	`#2C313A`	secondary text
`--fs-ash`	`#8A8474`	`#5C6068`	muted text
`--fs-moss`	`#5A7563`	(unchanged)	secondary action bg
`--fs-bronze`	`#8C6A3F`	(unchanged)	warning action bg
`--fs-oxblood`	`#7A2E2E`	(unchanged)	destructive action bg
`--fs-warning`	`#C99A4A`	(unchanged)	semantic marker
`--fs-error`	`#A04848`	(unchanged)	semantic marker
`--fs-info`	`#5A7A8A`	(unchanged)	semantic marker
`--fs-accent`	`#4A6B5C`	(unchanged)	brand accent
`--fs-on-action`	`#E8E2D2`	(unchanged)	action button label

Design rationale: light palette is "parchment / aged paper" — warm cream surfaces, dark text. Reads as an old book opened on a daylit desk; aligns with the FabledSword mythic register and preserves the bookish tone in light mode. Action and semantic colors are mid-tone earth/jewel hues that read on both light and dark backgrounds.

Files touched

Modify:

web/src/lib/styles/tokens.json — restructure colors into dark/light/flat sub-maps; add on-action to flat.
web/scripts/tokens-to-css.js — emit dual :root + [data-theme="light"] blocks for colors; preserve other token sections in :root.
web/src/app.html — add inline FOUC script in <head>.
web/src/routes/settings/+page.svelte — add Appearance card with 3-way theme selector.
Action button label class audit — find every site using text-fs-parchment on a bg-fs-moss / bg-fs-bronze / bg-fs-oxblood button and replace with text-fs-on-action. Expected count: 5–10 sites.

Create:

web/src/lib/stores/theme.svelte.ts — theme preference + resolved-theme rune store with matchMedia listener and localStorage persistence.

Generated (do not hand-edit):

web/src/lib/styles/tokens.generated.css — regenerated by npm run tokens (or whatever script invokes tokens-to-css.js).

Edge cases

Action button text contrast — addressed by --fs-on-action non-flipping token. Audit during implementation.
Cover-art images — external content, doesn't theme. Eyeball the missing-cover placeholder/glyph fallback in light mode.
Focus rings — grep for ring-offset-fs-*; if any uses a flipping token, switch to a non-flipping equivalent or ring-offset-background.
Cross-tab sync — out of scope. Theme change in tab A doesn't update tab B until reload. Documented limitation.
SSR — server doesn't know preference; inline head script applies it client-side before paint.
Error pages — SvelteKit +error.svelte and framework error fallback inherit data-theme attribute. No special handling.
localStorage unavailable — try/catch around localStorage read; falls back to default system.

Testing

Per project memory rule (no in-task tests; CI verifies), implementation tasks do not run tests. Plan does include the following test files for CI to execute:

Token generator unit test (web/scripts/tokens-to-css.test.js) — feed sample tokens.json with dark/light/flat, assert emitted CSS contains :root block with dark + flat values, and [data-theme="light"] block with light values only.
Theme store test (web/src/lib/stores/theme.svelte.test.ts) — mock matchMedia and localStorage. Verify (a) default is system resolving via matchMedia, (b) setTheme('light') persists + updates resolved, (c) matchMedia change event updates resolvedTheme when in system mode.
Settings Appearance card test (web/src/routes/settings/Appearance.test.ts) — render, click each segment, assert localStorage write + data-theme attribute on <html>.

Manual verification checklist (pre-merge eyeball pass): home, library, search, playlists list, playlist detail, queue, history, settings, admin (users / library / quarantine / lidarr) — all in both modes.

Implementation order (informs plan)

Restructure tokens.json (no behavior change yet — generator still emits one block).
Update generator to emit dual blocks.
Regenerate tokens.generated.css. Verify no visual regression in dark mode.
Add --fs-on-action flat token. Audit + swap action button label classes.
Create theme.svelte.ts store.
Add FOUC script to app.html.
Add Appearance card to Settings page, wire to store.
Manual eyeball pass in both modes; fix any contrast/border surprises.

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