diff --git a/CLAUDE.md b/CLAUDE.md
index 74bb300..5b5181a 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -78,3 +78,4 @@ Filter to a specific package with `-F`:
 - [docs/tech-stack.md](docs/tech-stack.md)
 - [docs/testing-guide.md](docs/testing-guide.md)
 - [docs/testing.md](docs/testing.md)
+- [docs/user-settings.md](docs/user-settings.md)
diff --git a/docs/user-settings.md b/docs/user-settings.md
new file mode 100644
index 0000000..7d8d464
--- /dev/null
+++ b/docs/user-settings.md
@@ -0,0 +1,92 @@
+# User Settings
+
+How Heim stores per-user preferences that don't belong to the domain model — things like color scheme, locale, or UI density. Distinct from auth state (session, identity) and from domain state (households, events).
+
+## Scope
+
+A "user setting" is a per-user preference that:
+
+- Affects presentation or client behavior, not domain decisions.
+- Is acceptable to lose if a user clears their browser (it's a preference, not data).
+- Contains no PII. PII belongs in forgettable payloads, never here.
+
+First setting: **color scheme**. The same shape applies to future settings like `locale` or `density`.
+
+## Client storage
+
+### localStorage keys
+
+- One key per setting. Key name is a kebab-case noun, preferring the matching CSS or web platform term where one exists.
+- Color scheme: key `color-scheme`, values `'light' | 'dark' | 'system'`. Default when unset: `'system'`.
+- Future examples (illustrative): `locale` (BCP 47 tag), `density` (`'comfortable' | 'compact'`).
+
+### DOM exposure
+
+Settings that affect first render are applied to `<html>` via dataset attributes so CSS can key off them without a paint cycle:
+
+- `<html data-{name}-pref="...">` holds the user's chosen preference, including any `'system'`/auto sentinel.
+- `<html data-{name}="...">` holds the resolved value, if resolution differs from the raw preference (e.g. `'system'` → `'light'`/`'dark'`).
+
+For color scheme this means `data-color-scheme-pref` (light/dark/system) and `data-color-scheme` (light/dark). The CSS `color-scheme` property on `<html>` is also set to the resolved value so native UA widgets follow along.
+
+### Pre-paint IIFE
+
+Settings that would cause a flash if applied after React mounts must be applied by an inline `<script>` in `<head>`, before any blocking stylesheet. The script is small, dependency-free, and wrapped in `try/catch` because `localStorage` throws in some privacy modes. If it throws, the CSS `<meta name="color-scheme" content="light dark" />` fallback handles UA widget colors and the page falls back to the `prefers-color-scheme` media query.
+
+The color-scheme IIFE lives in [`packages/web/index.html`](../packages/web/index.html).
+
+### React access
+
+Components read the current preference from `<html>`'s dataset (single source of truth, already applied) and write through both `localStorage.setItem` and the dataset attribute when the user changes the setting. No React-side hydration is required; the IIFE has already run.
+
+The component-side API is not yet implemented — current scope is the IIFE and this spec.
+
+## Server-side persistence (deferred)
+
+Today, settings live only on the device. The design below is what we'll implement when we want preferences to follow a user across devices.
+
+### Domain shape
+
+Per-user settings hang off the `User` aggregate as a non-PII blob on `UserState` (see [`packages/domain/src/user/user-state.ts`](../packages/domain/src/user/user-state.ts)):
+
+```ts
+type UserSettings = {
+  colorScheme?: "light" | "dark" | "system";
+  // future: locale, density, ...
+};
+
+type UserState = {
+  // ...existing identity fields...
+  settings: UserSettings;
+  settingsUpdatedAt: string; // ISO timestamp
+};
+```
+
+New event:
+
+```ts
+type UserSettingsUpdated = {
+  type: "UserSettingsUpdated";
+  settings: Partial<UserSettings>;
+  updatedAt: string;
+};
+```
+
+Apply by shallow merge into `UserState.settings`, replacing `settingsUpdatedAt`.
+
+### Sync
+
+- `GET /api/auth/session` response gains an optional `settings` field and `settingsUpdatedAt`.
+- On login (or session restore), if `settingsUpdatedAt` from the server is newer than the client's locally stored `settingsUpdatedAt` (a sibling localStorage key, e.g. `settings-updated-at`), overwrite localStorage from the server. Otherwise leave the client values alone.
+- When the user changes a setting in the UI, write locally first, then `POST` to a settings endpoint that emits a `UserSettingsUpdated` event. Reconciliation is last-write-wins by `settingsUpdatedAt`.
+
+### Why not cookies for hydration
+
+We considered putting the preference in a cookie so the server could deliver it before any client JS runs. Heim's web is a pure SPA today (Vite + React Router `BrowserRouter`); the HTML is a static asset built at compile time, identical for every request. A cookie wouldn't reach the document before the inline IIFE runs — the IIFE is already first-paint-fast — and there's no server-rendered HTML in which to inject a `data-color-scheme` attribute. If we later move to SSR (React Router framework mode) or have the API serve and template the HTML, revisit this: a cookie + server-injected `data-color-scheme` would let us drop the IIFE for logged-in users.
+
+## Privacy and erasure
+
+- `UserSettings` must contain no PII. Names, emails, addresses, free-text fields, or anything that could identify a person belong in the encrypted forgettable payload, not here.
+- A user's settings are erased alongside the rest of their data via the existing crypto-shredding path. The blob sits inside the user's forgettable area so right-to-erasure deletes it with everything else.
+- Aggregates of settings (e.g. "how many users prefer dark mode") are fine to derive from event projections; raw `UserSettingsUpdated` events are safe to keep because they hold no PII.
+- Before adding any new field to `UserSettings`, confirm against the PII rule in the root [`CLAUDE.md`](../CLAUDE.md). When in doubt, it's not a setting — it's profile data, and it belongs in the forgettable payload.
diff --git a/packages/web/index.html b/packages/web/index.html
index 7589118..9dea733 100644
--- a/packages/web/index.html
+++ b/packages/web/index.html
@@ -5,6 +5,21 @@
     <meta name="viewport" content="width=device-width, initial-scale=1.0" />
     <meta name="color-scheme" content="light dark" />
     <title>Heim</title>
+    <script>
+      (function () {
+        try {
+          var pref = localStorage.getItem("color-scheme") || "system";
+          var systemDark = window.matchMedia("(prefers-color-scheme: dark)").matches;
+          var resolved = pref === "system" ? (systemDark ? "dark" : "light") : pref;
+          var root = document.documentElement;
+          root.dataset.colorSchemePref = pref;
+          root.dataset.colorScheme = resolved;
+          root.style.colorScheme = resolved;
+        } catch (_) {
+          // localStorage can throw in private modes; CSS media query default applies.
+        }
+      })();
+    </script>
   </head>
   <body>
     <div id="root"></div>
