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docs: add design doc for ATB-53 theme resolution and server-side token injection
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docs/plans/2026-03-04-atb53-theme-resolution-design.md
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docs/plans/2026-03-04-atb53-theme-resolution-design.md
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# ATB-53: Theme Resolution and Server-Side Token Injection — Design
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**Status:** Approved
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**Linear:** [ATB-53](https://linear.app/atbb/issue/ATB-53)
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**Branch:** `atb-59-theme-editor` (active) → target `main`
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---
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## Context
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Dependencies are complete:
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- ATB-51 (lexicons): Done
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- ATB-52 (CSS token system): Done
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- ATB-55 (theme read API): Done
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- ATB-57 (theme write API): Done
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`tokensToCss()` and the two preset JSONs (`neobrutal-light.json`, `neobrutal-dark.json`) already exist in `apps/web/src/`. `BaseLayout` currently hardcodes neobrutal-light as a static module-level constant — this ticket makes it dynamic.
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---
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## Architecture
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### Middleware approach
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Hono middleware applied to `webRoutes` before all page routes. Uses Hono's typed Variables system (`Hono<{ Variables: { theme: ResolvedTheme } }>`). All route factories update to `new Hono<WebAppEnv>()` for end-to-end type safety.
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### Resolution waterfall
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```
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1. User preference — always null for now (TODO: populate when
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space.atbb.membership.preferredTheme is added in Theme Phase 4)
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2. Color scheme detection — atbb-color-scheme cookie → Sec-CH-Prefers-Color-Scheme hint → "light"
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3. Forum default — fetch GET /api/theme-policy, pick defaultLightThemeUri or
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defaultDarkThemeUri, fetch full theme, CID integrity check
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4. Hardcoded fallback — FALLBACK_THEME (neobrutal-light tokens + Google Fonts URL)
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```
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Any error at any step catches, logs, and returns `FALLBACK_THEME` with the detected color scheme.
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### CID integrity check
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The theme policy stores an expected CID per available theme. When the resolution waterfall fetches a theme by rkey, it compares `theme.cid` (returned by `GET /api/themes/:rkey`) against the CID stored in `policy.availableThemes`. A mismatch means the theme record changed after the policy was last updated — fall back to hardcoded and log a warning. Requires adding `cid` to `serializeThemeFull` in the AppView (small change, column already exists).
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### Caching
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Not in scope for ATB-53. ATB-56 adds the in-memory caching layer with cache invalidation on theme writes. When ATB-56 is implemented, `Vary: Cookie` headers should be added to HTML responses to prevent CDN caching from serving the wrong theme to users.
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---
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## Types
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```typescript
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// apps/web/src/lib/theme-resolution.ts
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export type ResolvedTheme = {
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tokens: Record<string, string>;
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cssOverrides: string | null;
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fontUrls: string[] | null;
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colorScheme: "light" | "dark";
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};
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export type WebAppEnv = {
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Variables: { theme: ResolvedTheme };
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};
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export const FALLBACK_THEME: ResolvedTheme = {
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tokens: neobrutalLight,
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cssOverrides: null,
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fontUrls: ["https://fonts.googleapis.com/css2?family=Space+Grotesk:wght@400;700&display=swap"],
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colorScheme: "light",
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};
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```
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---
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## BaseLayout changes
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`resolvedTheme: ResolvedTheme` becomes a required prop. Removes:
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- Module-level `ROOT_CSS` constant and `neobrutal-light` import
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- Hardcoded Google Fonts `<link>` tags (moved to `FALLBACK_THEME.fontUrls`)
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Adds:
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- `<meta http-equiv="Accept-CH" content="Sec-CH-Prefers-Color-Scheme" />`
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- Dynamic `:root { ... }` `<style>` block from `tokensToCss(resolvedTheme.tokens)`
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- `resolvedTheme.fontUrls` rendered as `<link rel="stylesheet">` tags (with `<link rel="preconnect">` to googleapis if any font URLs are present)
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- `resolvedTheme.cssOverrides` rendered as an additional `<style>` block if non-null
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---
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## Files Changed
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| File | Change |
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|------|--------|
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| `apps/appview/src/routes/themes.ts` | Add `cid` to `serializeThemeFull` |
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| `apps/web/src/lib/theme-resolution.ts` | **New** — `ResolvedTheme`, `WebAppEnv`, `FALLBACK_THEME`, `detectColorScheme`, `parseRkeyFromUri`, `resolveTheme` |
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| `apps/web/src/middleware/theme.ts` | **New** — Hono middleware wrapping `resolveTheme` |
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| `apps/web/src/routes/index.ts` | Add `WebAppEnv` type + `createThemeMiddleware` before routes |
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| `apps/web/src/layouts/base.tsx` | Dynamic theme prop, `Accept-CH`, font URLs, cssOverrides |
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| `apps/web/src/routes/home.tsx` | `new Hono<WebAppEnv>()`, pass theme to BaseLayout |
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| `apps/web/src/routes/boards.tsx` | Same |
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| `apps/web/src/routes/topics.tsx` | Same |
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| `apps/web/src/routes/login.tsx` | Same |
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| `apps/web/src/routes/new-topic.tsx` | Same |
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| `apps/web/src/routes/admin.tsx` | Same |
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| `apps/web/src/routes/not-found.tsx` | Same |
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| `apps/web/src/routes/auth.ts` | `new Hono<WebAppEnv>()` only (no HTML rendering) |
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| `apps/web/src/routes/mod.ts` | Same |
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---
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## Tests
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- `apps/web/src/lib/__tests__/theme-resolution.test.ts` (new): all waterfall branches — policy fetch failure, missing default URI, CID mismatch, happy path light, happy path dark, network error fallback; `detectColorScheme` priority order
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- `apps/web/src/layouts/__tests__/base.test.tsx`: update to pass `resolvedTheme` prop; verify `Accept-CH` meta; verify `cssOverrides` block renders when present
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- `apps/appview/src/routes/__tests__/themes.test.ts`: assert `cid` field in `GET /api/themes/:rkey` response