docs: write down the second ground, and the rule it creates

PRD gets F15 and the fork that asked for it, DATA-MODEL the prefs field
and the fact that a script outside the bundle now reads that key, and
ARCHITECTURE the pre-paint script — the one piece of this that is neither
CSS nor React.

The AGENTS rule is the part worth reading twice: a component that names a
colour instead of a token now looks right in one theme and wrong in the
other, and nothing will tell you which.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

AGENTS.md

@@ -115,9 +115,10 @@ src/client/       React app, service worker, manifest
                   lens.ts — who sees which rows (focus, outstanding, next-to-expire); pure
                   zoom.ts — the timeline's scale ladder; pure
                   lanes.ts — how the timeline stacks: a lane per game, or one deadline queue; pure
+                  theme.ts — dark or light, and what a game hue reads as on each
 scripts/          build-feed.ts, build-static.ts, parse-fixture.ts (offline), refresh-sources.ts (fetches)
 serve.ts          static server + /api/health
-test/             607 tests
+test/             620 tests
 fixtures/<game>/  raw HTML + .expected.json per source — pinned, kept forever
 snapshots/        current page per source, rewritten by refresh — see its README
 ```
@@ -510,6 +511,18 @@ to an open page). Four things hold it up:
 - Keep old fixtures when a source changes shape — the old one is the regression test proving the
   parser still handles the previous format. Fixtures are pinned and permanent; `snapshots/` is the
   current page and gets overwritten. Do not conflate them.
+- **Colour is a token, and every token has two answers.** The app ships dark and offers light
+  (PRD F15), and the whole difference is a set of custom properties re-struck under
+  `:root[data-theme="light"]` in `styles.css`. So a component names `ink`, `hairline` or `soon` and
+  never a colour: a literal — `bg-white`, a hex, a hardcoded scrim — is a component that looks
+  right in one theme and wrong in the other, and nothing will tell you which. The two things that
+  genuinely cannot be tokens are the game hues, which are *data* (`games.ts`, and the reader's own),
+  and the pre-paint script in `index.html`; both are handled in `src/client/state/theme.ts` and
+  pinned by `test/theme.test.ts`.
+  Two rules follow. **The heat ramp must stay readable on whichever ground it is on** — it carries
+  meaning, and the dark theme's amber is 1.9:1 on paper, so each step is re-struck rather than
+  reused. And **the dark theme does not move**: `readableHue` returns dark untouched by
+  construction, because adding a theme is not a licence to redraw the one that shipped.
 - **The page is two columns past `lg`, and the split is the one below.** What the page *tells* the
   reader to do — the next deadlines, tonight's dailies — pins to a rail on the left and stays put
   while the lists it *shows* them scroll beside it. Below that breakpoint it is one column in the

docs/ARCHITECTURE.md

@@ -120,7 +120,8 @@ src/
       useMarkSet.ts     ignores (and the superseded completions shape)
       useProgress.ts    status, effort, note, daily override  (F12)
       useDailyLog.ts    which game-days are ticked off
-      usePrefs.ts       region, filters, focus, view, onboarding flags
+      usePrefs.ts       region, filters, focus, view, theme, onboarding flags
+      theme.ts          dark/light: resolving it, applying it, hues on paper
       useCustom.ts      the reader's own games and events     (F13)
       gameMeta.tsx      lane id → name, label, hue; resolves custom lanes too
       sort.ts           deadline order, or what you're partway through
@@ -239,6 +240,24 @@ back to the last copy seen). Countdowns run off the device clock, so the app sta
 network. Offline state is surfaced in the header and above the footer — stale data must never be
 presented as current.
 
+### The theme, before the bundle arrives
+
+The page is drawn dark by default and light when the reader has asked for it (PRD F15). Which one
+is decided by one attribute on `<html>`: `styles.css` holds the dark tokens on `:root` and
+re-strikes them under `:root[data-theme="light"]`, so nothing in React knows a theme exists and no
+component holds a colour of its own.
+
+Setting that attribute is the one part React cannot do in time. It mounts after the bundle has
+downloaded and parsed, which on a cold cache is long enough to show a reader who chose light a
+dark page, on every single load. So a small inline script in `index.html` reads the same
+`localStorage` prefs key the app does, sets the same attribute, and updates `<meta
+name="theme-color">` — before first paint, and with a `try`/`catch` so storage being unavailable
+costs the reader the default theme rather than the page.
+
+That makes the theme's ground colour a fact written in three files that cannot import each other:
+the stylesheet, `state/theme.ts` (which needs it for the meta tag), and the shell. `test/theme.test.ts`
+pins the three together rather than trusting them to be edited at the same time.
+
 ### Shipping a new version to an open page
 
 A cache-first shell is what makes the offline story work and what makes a deploy invisible: the

docs/DATA-MODEL.md

@@ -219,7 +219,7 @@ Namespaced, versioned, and small. Nothing here ever goes to the server.
 "gacha-tracker:v1:ignored"      // { [eventId]: { at } }  — "stop showing me this"
 "gacha-tracker:v1:prefs"        // { region, hiddenGames[], knownGames[]?, focusGame, sort, view,
                                 //   timelineDayWidth, timelineGroup, detectDaily, showCompleted,
-                                //   showIgnored, regionConfirmed, onboarded }
+                                //   showIgnored, theme, regionConfirmed, onboarded }
                                 // timelineDayWidth is px per day on the board, stored as the
                                 // measurement rather than a step number and read through
                                 // snapDayWidth — so a value from an older ladder still opens
@@ -230,6 +230,10 @@ Namespaced, versioned, and small. Nothing here ever goes to the server.
                                 // knownGames is every lane the reader has been offered. Absent
                                 // means unrecorded, not "offered nothing" — see PRD F8; a lane
                                 // missing from it is new to them and arrives switched off.
+                                // theme is "dark" | "light" | "system", defaulting to dark — see
+                                // PRD F15. It is read by the app *and* by a pre-paint script in
+                                // index.html, which is the only thing outside the client bundle
+                                // that touches a key in this space.
 "gacha-tracker:v1:completions"  // SUPERSEDED — read once to migrate, never written
 ```
 

docs/PRD.md

@@ -284,6 +284,37 @@ The footer shows when the feed was last updated, per game. If a game's data is m
 stale, its lane carries a warning badge. Never present stale data as current — the whole value
 proposition is trust in the dates.
 
+**F15 — Light mode, with dark still the default.**
+The app is a lit instrument panel and that is what it should be on first sight, but it is also read
+on a train in daylight and by people who find a dark UI harder rather than moodier. A public fork
+worked a theme toggle out first and is credited for the idea in the colophon. So the ground is
+the reader's choice: **Dark**, **Light**, or **System** to follow the device, in settings next to the
+region — both are "how do I read this?" and neither changes what the page knows.
+
+Dark is what it ships as, and deliberately not `System`. A reader whose laptop is in light mode has
+said something about their laptop, not about this page; defaulting to the device would also move
+every existing reader the first time they loaded a build that had this, which is the `knownGames`
+mistake in a different costume (F8). Choosing `System` is one tap, and from then on it *is* their
+answer.
+
+Three things the light theme has to get right, because they are what a second palette usually gets
+wrong:
+
+- **It is a re-strike, not an inversion.** Every colour in the UI is a token, and light redefines the
+  tokens rather than adding a second set of rules to components. Nothing in the app asks which theme
+  it is in, so a new component cannot forget to support one.
+- **Urgency still reads.** The heat ramp carries meaning, and the dark theme's amber is 1.9:1 on
+  paper — an urgency the reader cannot read is not urgency. Every step is re-struck to clear 4.5:1
+  against the light ground, keeping its order and its meaning.
+- **A game keeps its colour.** The hues are identity (F1) and were all picked against a near-black
+  ground; on paper the bright ones vanish. They are darkened until they read, along the same hue, so
+  Wuthering Waves is still the green one — including the hues a reader picked for a game they
+  invented (F13).
+
+Switching is instant, costs nothing and saves nothing: no reload, and nothing marked, typed or
+ticked is touched. And it survives the load it is chosen on — the shell sets the theme before first
+paint, so a reader on light is never shown a dark page while the bundle downloads.
+
 ## Out of scope for v1
 
 Accounts and sync; push notifications; in-game resource or pull tracking; native mobile apps (the