diff --git a/.agents/conventions.md b/.agents/conventions.md
index 46e1fcb..1bf6a04 100644
--- a/.agents/conventions.md
+++ b/.agents/conventions.md
@@ -596,6 +596,7 @@ Theme is managed by `src/lib/theme.tsx` which provides `ThemeProvider` and `useT
 - **Flash prevention:** Inline `<script>` in `<head>` reads localStorage before React hydrates.
 - **System detection:** `prefers-color-scheme` media query listener when preference is `"system"`.
 - **Default:** `"dark"` (existing users who haven't set a preference get dark mode).
+- **localStorage safety:** All `localStorage` calls must be wrapped in try-catch. Browsers may throw `SecurityError` when storage access is denied (privacy settings, embedded contexts, enterprise policies). Fall back to `"dark"` on read, silently ignore on write.
 
 ```typescript
 // Reading theme in a client component
diff --git a/src/lib/sentry/sentry.test.ts b/src/lib/sentry/sentry.test.ts
index 9658850..27cfcb8 100644
--- a/src/lib/sentry/sentry.test.ts
+++ b/src/lib/sentry/sentry.test.ts
@@ -20,6 +20,7 @@ const BARE_CATCH_PERMANENT_ALLOWLIST = new Set([
   "src/components/members/invite-form.tsx", // clipboard writeText — intentionally silent on permission denied
   "src/components/members/pending-invite-list.tsx", // clipboard writeText — intentionally silent on permission denied
   "src/lib/use-persisted-expanded.ts", // localStorage read/write — intentionally silent in private browsing / SSR
+  "src/lib/theme.tsx", // localStorage read/write — intentionally silent when storage access is denied (SecurityError)
   "src/components/editor/demo-editor.tsx", // URL validation (new URL() throws) — same pattern as editor.tsx
   "src/components/editor/local-persistence-plugin.tsx", // sessionStorage read/write — intentionally silent in private browsing
 ]);
diff --git a/src/lib/theme.test.tsx b/src/lib/theme.test.tsx
new file mode 100644
index 0000000..abadefc
--- /dev/null
+++ b/src/lib/theme.test.tsx
@@ -0,0 +1,91 @@
+import { describe, it, expect, vi, beforeEach, afterEach } from "vitest";
+import { renderHook, act } from "@testing-library/react";
+import type { ReactNode } from "react";
+import { ThemeProvider, useTheme } from "./theme";
+
+function wrapper({ children }: { children: ReactNode }) {
+  return <ThemeProvider>{children}</ThemeProvider>;
+}
+
+describe("ThemeProvider", () => {
+  beforeEach(() => {
+    localStorage.clear();
+    document.documentElement.removeAttribute("data-theme");
+    document.documentElement.classList.remove("dark");
+  });
+
+  it("defaults to dark when localStorage is empty", () => {
+    const { result } = renderHook(() => useTheme(), { wrapper });
+    expect(result.current.preference).toBe("dark");
+    expect(result.current.resolved).toBe("dark");
+  });
+
+  it("reads stored preference from localStorage", () => {
+    localStorage.setItem("memo-theme", "light");
+    const { result } = renderHook(() => useTheme(), { wrapper });
+    expect(result.current.preference).toBe("light");
+    expect(result.current.resolved).toBe("light");
+  });
+
+  it("ignores invalid stored values", () => {
+    localStorage.setItem("memo-theme", "invalid-value");
+    const { result } = renderHook(() => useTheme(), { wrapper });
+    expect(result.current.preference).toBe("dark");
+  });
+
+  it("persists preference changes to localStorage", () => {
+    const { result } = renderHook(() => useTheme(), { wrapper });
+    act(() => result.current.setPreference("light"));
+    expect(localStorage.getItem("memo-theme")).toBe("light");
+    expect(result.current.preference).toBe("light");
+  });
+
+  /**
+   * Regression test for Sentry MEMO-2H: SecurityError when localStorage is
+   * blocked (restricted browsers, privacy settings, embedded contexts).
+   * ThemeProvider must fall back to dark theme instead of crashing.
+   */
+  describe("localStorage SecurityError handling", () => {
+    let getItemSpy: ReturnType<typeof vi.spyOn>;
+    let setItemSpy: ReturnType<typeof vi.spyOn>;
+
+    beforeEach(() => {
+      getItemSpy = vi.spyOn(Storage.prototype, "getItem").mockImplementation(
+        () => {
+          throw new DOMException(
+            "Failed to read the 'localStorage' property from 'Window': Access is denied for this document.",
+            "SecurityError",
+          );
+        },
+      );
+      setItemSpy = vi.spyOn(Storage.prototype, "setItem").mockImplementation(
+        () => {
+          throw new DOMException(
+            "Failed to set the 'localStorage' property on 'Window': Access is denied for this document.",
+            "SecurityError",
+          );
+        },
+      );
+    });
+
+    afterEach(() => {
+      getItemSpy.mockRestore();
+      setItemSpy.mockRestore();
+    });
+
+    it("falls back to dark theme when getItem throws SecurityError", () => {
+      const { result } = renderHook(() => useTheme(), { wrapper });
+      expect(result.current.preference).toBe("dark");
+      expect(result.current.resolved).toBe("dark");
+    });
+
+    it("does not crash when setItem throws SecurityError", () => {
+      const { result } = renderHook(() => useTheme(), { wrapper });
+      expect(() => {
+        act(() => result.current.setPreference("light"));
+      }).not.toThrow();
+      expect(result.current.preference).toBe("light");
+      expect(result.current.resolved).toBe("light");
+    });
+  });
+});
diff --git a/src/lib/theme.tsx b/src/lib/theme.tsx
index 2b8453d..35e81f8 100644
--- a/src/lib/theme.tsx
+++ b/src/lib/theme.tsx
@@ -28,17 +28,25 @@ function applyTheme(theme: ResolvedTheme) {
 export function ThemeProvider({ children }: { children: React.ReactNode }) {
   const [preference, setPreferenceState] = useState<ThemePreference>(() => {
     if (typeof window === "undefined") return "dark";
-    const stored = localStorage.getItem(STORAGE_KEY);
-    return stored === "light" || stored === "dark" || stored === "system"
-      ? stored
-      : "dark";
+    try {
+      const stored = localStorage.getItem(STORAGE_KEY);
+      return stored === "light" || stored === "dark" || stored === "system"
+        ? stored
+        : "dark";
+    } catch {
+      return "dark";
+    }
   });
   const [resolved, setResolved] = useState<ResolvedTheme>(() =>
     preference === "system" ? getSystemTheme() : preference,
   );
 
   function setPreference(pref: ThemePreference) {
-    localStorage.setItem(STORAGE_KEY, pref);
+    try {
+      localStorage.setItem(STORAGE_KEY, pref);
+    } catch {
+      // Storage unavailable (SecurityError in restricted browsers)
+    }
     setPreferenceState(pref);
     const next = pref === "system" ? getSystemTheme() : pref;
     setResolved(next);
