# Dark-Mode Theme Toggle Implementation Plan > **For agentic workers:** REQUIRED SUB-SKILL: Use superpowers:subagent-driven-development (recommended) or superpowers:executing-plans to implement this plan task-by-task. Steps use checkbox (`- [ ]`) syntax for tracking. **Goal:** Ship a tri-state (Light/Dark/System) theme toggle that activates the existing `.dark` token set, persists to `localStorage`, defaults to System (live-tracking the OS), and never flashes on reload. **Architecture:** Client-only theming over CSS custom properties — no new dependency. A framework-free core (`theme.ts`) resolves/reads/applies the theme; a `useTheme` hook mirrors `use-locale`; a synchronous inline script in `index.html` applies the class before first paint; an icon segmented `ThemeSwitch` lives in the header next to `LangSwitch`. The `.dark` class on `` activates the dark tokens migrated in #49. **Tech Stack:** React 19 + TS + pnpm, Tailwind v4 (OKLCH tokens in `index.css`), lucide-react (already a dep), Vitest + RTL + MSW + Storybook. Test runner: `pnpm test` (vitest, single pass). **Conventions:** pnpm; **no `any`/`eslint-disable`/`@ts-ignore`**; no codename; en/sv parity; source double-quote/semicolon, stories single-quote/no-semicolon; token classes only (no raw colors — `check:colors` must pass); guard DOM globals (`window`/`localStorage`/`matchMedia`/`document`) for jsdom/test safety. **Spec:** `docs/superpowers/specs/2026-06-07-dark-mode-theme-toggle-design.md` **File structure:** - `web/src/theme/theme.ts` (new) — `THEME_KEY`, `Theme`, `resolveTheme`, `readTheme`, `applyTheme`. - `web/src/theme/theme.test.ts` (new) — unit tests for the core. - `web/src/theme/use-theme.ts` (new) — `useTheme()` hook. - `web/src/shell/theme-switch.tsx` (new) — the icon segmented control. - `web/src/shell/theme-switch.test.tsx` (new) — interaction tests. - `web/src/shell/theme-switch.stories.tsx` (new) — Storybook story. - `web/src/shell/app-shell.tsx` (modify) — mount ``. - `web/src/i18n/en.json`, `web/src/i18n/sv.json` (modify) — `theme.*` keys. - `web/index.html` (modify) — inline FOUC-prevention script. - `web/src/index.css` (modify) — dark `--primary`/`--ring` contrast tweak. --- # Task 1: Theme core (`theme.ts`) + unit tests **Files:** - Create: `web/src/theme/theme.ts` - Create: `web/src/theme/theme.test.ts` - [ ] **Step 1: Write the failing tests** — `web/src/theme/theme.test.ts`: ```ts import { afterEach, expect, test, vi } from "vitest"; import { applyTheme, readTheme, resolveTheme, THEME_KEY } from "./theme"; function mockMatchMedia(matches: boolean) { vi.stubGlobal("matchMedia", (query: string) => ({ matches, media: query, onchange: null, addEventListener: vi.fn(), removeEventListener: vi.fn(), addListener: vi.fn(), removeListener: vi.fn(), dispatchEvent: vi.fn(), })); } afterEach(() => { vi.unstubAllGlobals(); localStorage.clear(); document.documentElement.classList.remove("dark"); }); test("resolveTheme returns explicit values verbatim", () => { expect(resolveTheme("light")).toBe("light"); expect(resolveTheme("dark")).toBe("dark"); }); test("resolveTheme maps system via prefers-color-scheme", () => { mockMatchMedia(true); expect(resolveTheme("system")).toBe("dark"); mockMatchMedia(false); expect(resolveTheme("system")).toBe("light"); }); test("readTheme defaults to system when unset or invalid", () => { expect(readTheme()).toBe("system"); localStorage.setItem(THEME_KEY, "bogus"); expect(readTheme()).toBe("system"); localStorage.setItem(THEME_KEY, "dark"); expect(readTheme()).toBe("dark"); }); test("applyTheme toggles the dark class on documentElement", () => { mockMatchMedia(false); applyTheme("dark"); expect(document.documentElement.classList.contains("dark")).toBe(true); applyTheme("light"); expect(document.documentElement.classList.contains("dark")).toBe(false); mockMatchMedia(true); applyTheme("system"); expect(document.documentElement.classList.contains("dark")).toBe(true); }); ``` - [ ] **Step 2: Run to verify it fails** Run: `cd web && pnpm vitest run src/theme/theme.test.ts` Expected: FAIL — cannot import from `./theme` (module not found). - [ ] **Step 3: Implement** — `web/src/theme/theme.ts`: ```ts export const THEME_KEY = "theme"; export type Theme = "light" | "dark" | "system"; const THEMES: readonly Theme[] = ["light", "dark", "system"]; function prefersDark(): boolean { return ( typeof window !== "undefined" && typeof window.matchMedia === "function" && window.matchMedia("(prefers-color-scheme: dark)").matches ); } export function resolveTheme(theme: Theme): "light" | "dark" { if (theme === "light" || theme === "dark") return theme; return prefersDark() ? "dark" : "light"; } export function readTheme(): Theme { if (typeof localStorage === "undefined") return "system"; const stored = localStorage.getItem(THEME_KEY); return THEMES.includes(stored as Theme) ? (stored as Theme) : "system"; } export function applyTheme(theme: Theme): void { if (typeof document === "undefined") return; document.documentElement.classList.toggle("dark", resolveTheme(theme) === "dark"); } ``` - [ ] **Step 4: Run to verify it passes** Run: `cd web && pnpm vitest run src/theme/theme.test.ts` Expected: PASS (4 tests). - [ ] **Step 5: Commit** ```bash git add web/src/theme/theme.ts web/src/theme/theme.test.ts git commit -m "feat(web): theme core — resolve/read/apply tri-state theme (#59)" ``` --- # Task 2: `useTheme` hook **Files:** - Create: `web/src/theme/use-theme.ts` (No standalone unit test — the hook is exercised by `theme-switch.test.tsx` in Task 3, which drives it through real UI per the project's testing style. `theme.ts` carries the logic and is unit-tested in Task 1.) - [ ] **Step 1: Implement** — `web/src/theme/use-theme.ts`: ```ts import { useEffect, useState } from "react"; import { applyTheme, readTheme, type Theme } from "./theme"; export function useTheme(): { theme: Theme; setTheme: (theme: Theme) => void } { const [theme, setThemeState] = useState(readTheme); const setTheme = (next: Theme) => { if (typeof localStorage !== "undefined") localStorage.setItem("theme", next); setThemeState(next); applyTheme(next); }; useEffect(() => { applyTheme(theme); if (theme !== "system") return; if (typeof window === "undefined" || typeof window.matchMedia !== "function") return; const mql = window.matchMedia("(prefers-color-scheme: dark)"); const onChange = () => applyTheme("system"); mql.addEventListener("change", onChange); return () => mql.removeEventListener("change", onChange); }, [theme]); return { theme, setTheme }; } ``` Note: import `THEME_KEY` from `./theme` and use it instead of the literal `"theme"` for the `localStorage.setItem` key (DRY with the core). Update the import line to `import { applyTheme, readTheme, THEME_KEY, type Theme } from "./theme";` and use `localStorage.setItem(THEME_KEY, next)`. - [ ] **Step 2: Typecheck** Run: `cd web && pnpm typecheck` Expected: PASS (no errors). - [ ] **Step 3: Commit** ```bash git add web/src/theme/use-theme.ts git commit -m "feat(web): useTheme hook with live system tracking (#59)" ``` --- # Task 3: `ThemeSwitch` UI + i18n + tests + story **Files:** - Create: `web/src/shell/theme-switch.tsx` - Create: `web/src/shell/theme-switch.test.tsx` - Create: `web/src/shell/theme-switch.stories.tsx` - Modify: `web/src/i18n/en.json`, `web/src/i18n/sv.json` - [ ] **Step 1: Add i18n keys.** In `web/src/i18n/en.json`, add a top-level `theme` namespace (place after the `labels` entry): ```json "theme": { "light": "Light", "dark": "Dark", "system": "System" }, ``` In `web/src/i18n/sv.json`, the matching entry: ```json "theme": { "light": "Ljust", "dark": "Mörkt", "system": "System" }, ``` - [ ] **Step 2: Write the failing test** — `web/src/shell/theme-switch.test.tsx`: ```tsx import { afterEach, beforeEach, expect, test, vi } from "vitest"; import { screen } from "@testing-library/react"; import userEvent from "@testing-library/user-event"; import { renderApp } from "../test/render"; import { ThemeSwitch } from "./theme-switch"; beforeEach(() => { vi.stubGlobal("matchMedia", (query: string) => ({ matches: false, media: query, onchange: null, addEventListener: vi.fn(), removeEventListener: vi.fn(), addListener: vi.fn(), removeListener: vi.fn(), dispatchEvent: vi.fn(), })); }); afterEach(() => { vi.unstubAllGlobals(); localStorage.clear(); document.documentElement.classList.remove("dark"); }); test("selecting Dark applies the dark class and persists", async () => { renderApp(); await userEvent.click(screen.getByRole("button", { name: /dark/i })); expect(document.documentElement.classList.contains("dark")).toBe(true); expect(localStorage.getItem("theme")).toBe("dark"); expect(screen.getByRole("button", { name: /dark/i })).toHaveAttribute( "aria-pressed", "true", ); }); test("selecting Light removes the dark class and persists", async () => { localStorage.setItem("theme", "dark"); renderApp(); await userEvent.click(screen.getByRole("button", { name: /light/i })); expect(document.documentElement.classList.contains("dark")).toBe(false); expect(localStorage.getItem("theme")).toBe("light"); }); test("selecting System resolves via prefers-color-scheme", async () => { vi.stubGlobal("matchMedia", (query: string) => ({ matches: true, media: query, onchange: null, addEventListener: vi.fn(), removeEventListener: vi.fn(), addListener: vi.fn(), removeListener: vi.fn(), dispatchEvent: vi.fn(), })); renderApp(); await userEvent.click(screen.getByRole("button", { name: /system/i })); expect(localStorage.getItem("theme")).toBe("system"); expect(document.documentElement.classList.contains("dark")).toBe(true); }); ``` - [ ] **Step 3: Run to verify it fails** Run: `cd web && pnpm vitest run src/shell/theme-switch.test.tsx` Expected: FAIL — cannot import `ThemeSwitch`. - [ ] **Step 4: Implement** — `web/src/shell/theme-switch.tsx`: ```tsx import { Monitor, Moon, Sun } from "lucide-react"; import { useTranslation } from "react-i18next"; import { useTheme } from "../theme/use-theme"; import type { Theme } from "../theme/theme"; import { cn } from "@/lib/utils"; const OPTIONS: { value: Theme; Icon: typeof Sun }[] = [ { value: "light", Icon: Sun }, { value: "dark", Icon: Moon }, { value: "system", Icon: Monitor }, ]; export function ThemeSwitch() { const { t } = useTranslation(); const { theme, setTheme } = useTheme(); return (
{OPTIONS.map(({ value, Icon }) => { const active = theme === value; return ( ); })}
); } ``` (Verify the `cn` import path matches the project — other `ui/*` files import `cn` from `@/lib/utils`. If `lib/utils` is absent, mirror whatever `button.tsx` uses.) - [ ] **Step 5: Run to verify it passes** Run: `cd web && pnpm vitest run src/shell/theme-switch.test.tsx` Expected: PASS (3 tests). - [ ] **Step 6: Write the Storybook story** — `web/src/shell/theme-switch.stories.tsx`: ```tsx import type { Meta, StoryObj } from '@storybook/react-vite' import { expect } from 'storybook/test' import { ThemeSwitch } from './theme-switch' const meta = { component: ThemeSwitch, tags: ['ai-generated'], } satisfies Meta export default meta type Story = StoryObj export const Default: Story = { play: async ({ canvas }) => { await expect(canvas.getByRole('button', { name: /light/i })).toBeInTheDocument() await expect(canvas.getByRole('button', { name: /dark/i })).toBeInTheDocument() await expect(canvas.getByRole('button', { name: /system/i })).toBeInTheDocument() }, } ``` (Note: the story exercises rendering only — it does not click options, to avoid mutating `` globally across the browser-mode test run.) - [ ] **Step 7: Run the story as a test + lint** Run: `cd web && pnpm vitest run src/shell/theme-switch.stories.tsx && pnpm lint` Expected: PASS. - [ ] **Step 8: Commit** ```bash git add web/src/shell/theme-switch.tsx web/src/shell/theme-switch.test.tsx web/src/shell/theme-switch.stories.tsx web/src/i18n/en.json web/src/i18n/sv.json git commit -m "feat(web): ThemeSwitch icon segmented control + theme.* i18n (#59)" ``` --- # Task 4: Mount in the header + FOUC inline script **Files:** - Modify: `web/src/shell/app-shell.tsx` - Modify: `web/index.html` - [ ] **Step 1: Mount `ThemeSwitch`.** In `web/src/shell/app-shell.tsx`, add the import: ```tsx import { ThemeSwitch } from "./theme-switch"; ``` and render it in the header immediately before ``: ```tsx
``` (Match the existing header's exact JSX; only insert the one line. Do not change other markup.) - [ ] **Step 2: Add the FOUC-prevention inline script.** In `web/index.html`, inside `` BEFORE the ` ``` - [ ] **Step 3: Verify the app-shell test still passes** (the header now has an extra control): Run: `cd web && pnpm vitest run src/shell/app-shell.test.tsx` Expected: PASS (the existing "language switch" test is unaffected — ThemeSwitch buttons have distinct accessible names). - [ ] **Step 4: Build to verify `index.html` is valid** Run: `cd web && pnpm build` Expected: built successfully (Vite processes the inline script). - [ ] **Step 5: Commit** ```bash git add web/src/shell/app-shell.tsx web/index.html git commit -m "feat(web): mount ThemeSwitch in header + pre-paint theme init (#59)" ``` --- # Task 5: Dark `--primary` contrast tweak + final verification **Files:** - Modify: `web/src/index.css` - [ ] **Step 1: Compute the new dark `--primary`.** The dark button label uses `--primary-foreground: oklch(0.205 0 0)` (near-black) on `--primary: oklch(0.673 0.182 276.935)` (~3.21:1). Lower the lightness (and keep it a recognizable indigo) until WCAG contrast vs `oklch(0.205 0 0)` is **≥4.5:1**. A good starting point is `oklch(0.62 0.20 277)`; compute the exact value with a contrast check (convert both to sRGB relative luminance, `(L1+0.05)/(L2+0.05) ≥ 4.5`). In the `.dark` block of `web/src/index.css`, update BOTH `--primary` and `--ring` (they must match) to the chosen value: ```css --primary: oklch( 277); ... --ring: oklch( 277); ``` Leave `--primary-foreground: oklch(0.205 0 0)` and the entire `:root` (light) block unchanged. - [ ] **Step 2: Verify the contrast.** State the computed ratio in the commit body (must be ≥4.5:1). Sanity-check the value is still visibly indigo (hue ~277, chroma not flattened to gray). - [ ] **Step 3: Full gate (single test pass).** Run: ```bash cd web && pnpm typecheck && pnpm lint && pnpm test && pnpm build && pnpm check:size && pnpm check:colors ``` Expected: all green. `check:colors` passes (icons are not color utilities). `check:size` within 250 KB gz (three lucide icons are negligible). Tests run exactly ONCE (no concurrent runs). - [ ] **Step 4: Codename + status checks.** ```bash git grep -in 'biggus\|dickus' -- web/src web/index.html; echo "codename-exit=$?" git status --short ``` Expected: no codename matches; working tree shows only intended changes. - [ ] **Step 5: Manual smoke (recommended).** `pnpm dev`, toggle Light/Dark/System; confirm the app switches, a dark reload doesn't flash light, primary buttons are legible in dark, and switching the OS theme while in System updates the app live. - [ ] **Step 6: Commit** ```bash git add web/src/index.css git commit -m "fix(web): raise dark --primary contrast to AA for button labels (#59)" ``` --- ## Self-Review (completed) **Spec coverage:** tri-state model + System default (T1 `resolveTheme`/`readTheme`, T3 UI); persisted to localStorage (T2 `setTheme`, T3 tests); `.dark` on `` (T1 `applyTheme`); live system tracking (T2 `useEffect` matchMedia listener); FOUC prevention (T4 inline script); icon segmented control next to LangSwitch (T3 + T4 mount); en/sv `theme.*` (T3); aria-pressed/aria-label (T3); dark `--primary` contrast ≥4.5:1 + `--ring` sync (T5); gate incl. check:colors/check:size + no codename + no new dep (T5). All acceptance criteria 1–6 mapped. ✓ **Placeholder scan:** the only "computed" value is the exact dark `--primary` OKLCH — a genuine WCAG measurement step with a concrete starting point and an explicit acceptance threshold (≥4.5:1), not a TODO. All code blocks are complete. ✓ **Type consistency:** `Theme` type defined in `theme.ts` (T1), imported by `use-theme.ts` (T2) and `theme-switch.tsx` (T3); `THEME_KEY` from `theme.ts` used in T2's setter; `resolveTheme`/`readTheme`/ `applyTheme` signatures consistent across tasks; i18n keys `theme.light/dark/system` defined in T3 and referenced by `t(\`theme.${value}\`)` in T3's component. ✓ ## Notes - No new dependency (lucide-react already present; `.dark` tokens already exist from #49). - The inline FOUC script is intentionally plain ES5-ish + try/catch — it runs before the bundle and must never throw. - Cross-tab sync and per-account/server theme default are explicit follow-ups (not in this plan).