diff --git a/client/src/lib/theme.test.ts b/client/src/lib/theme.test.ts
new file mode 100644
index 0000000..0c9eda4
--- /dev/null
+++ b/client/src/lib/theme.test.ts
@@ -0,0 +1,25 @@
+import { describe, it, expect, beforeEach } from "vitest";
+import { nextTheme, getInitialTheme } from "./theme.js";
+
+describe("nextTheme", () => {
+  it("bascule dark <-> light", () => {
+    expect(nextTheme("dark")).toBe("light");
+    expect(nextTheme("light")).toBe("dark");
+  });
+});
+
+describe("getInitialTheme", () => {
+  beforeEach(() => {
+    // @ts-expect-error - environnement node sans localStorage
+    delete globalThis.localStorage;
+  });
+  it("retombe sur dark sans localStorage", () => {
+    expect(getInitialTheme()).toBe("dark");
+  });
+  it("lit la valeur persistée si présente", () => {
+    const store: Record<string, string> = { "su-theme": "light" };
+    // @ts-expect-error - stub minimal
+    globalThis.localStorage = { getItem: (k: string) => store[k] ?? null };
+    expect(getInitialTheme()).toBe("light");
+  });
+});
diff --git a/client/src/lib/theme.ts b/client/src/lib/theme.ts
new file mode 100644
index 0000000..7ee6b40
--- /dev/null
+++ b/client/src/lib/theme.ts
@@ -0,0 +1,25 @@
+// client/src/lib/theme.ts
+export type Theme = "dark" | "light";
+const KEY = "su-theme";
+
+export function nextTheme(t: Theme): Theme {
+  return t === "dark" ? "light" : "dark";
+}
+
+export function getInitialTheme(): Theme {
+  try {
+    const v = globalThis.localStorage?.getItem(KEY);
+    return v === "light" ? "light" : "dark";
+  } catch {
+    return "dark";
+  }
+}
+
+export function applyTheme(t: Theme): void {
+  try {
+    document.documentElement.dataset.theme = t;
+    globalThis.localStorage?.setItem(KEY, t);
+  } catch {
+    /* localStorage indisponible (mode privé) : on ignore la persistance */
+  }
+}
diff --git a/vitest.config.ts b/vitest.config.ts
index ab14c0a..b4c1afa 100644
--- a/vitest.config.ts
+++ b/vitest.config.ts
@@ -3,7 +3,7 @@ import { defineConfig } from "vitest/config";
 export default defineConfig({
   test: {
     globals: true,
-    include: ["server/**/*.test.ts", "shared/**/*.test.ts"],
+    include: ["server/**/*.test.ts", "shared/**/*.test.ts", "client/**/*.test.ts"],
     environment: "node",
   },
   resolve: { alias: { "@shared": new URL("./shared", import.meta.url).pathname } },