diff --git a/apps/web/src/components/theme-toggle.test.tsx b/apps/web/src/components/theme-toggle.test.tsx
new file mode 100644
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+++ b/apps/web/src/components/theme-toggle.test.tsx
@@ -0,0 +1,149 @@
+import { describe, it, expect, vi, beforeEach } from "vitest";
+import { renderToStaticMarkup } from "react-dom/server";
+import type { ReactNode } from "react";
+
+// next-themes accesses window/localStorage on import; stub the hook so the
+// component is testable in a Node environment.
+const setThemeMock = vi.fn();
+const useThemeReturn: { theme: string | undefined; setTheme: typeof setThemeMock } = {
+  theme: "system",
+  setTheme: setThemeMock,
+};
+vi.mock("next-themes", () => ({
+  useTheme: () => useThemeReturn,
+}));
+
+// Radix DropdownMenu uses portals + client refs. Render trigger and items
+// inline so we can assert on the markup deterministically.
+vi.mock("@/components/ui/dropdown-menu", () => ({
+  DropdownMenu: ({ children }: { children: ReactNode }) => <>{children}</>,
+  DropdownMenuTrigger: ({ children }: { children: ReactNode; asChild?: boolean }) => <>{children}</>,
+  DropdownMenuContent: ({ children }: { children: ReactNode }) => (
+    <div data-testid="dropdown-content">{children}</div>
+  ),
+  DropdownMenuItem: ({
+    children,
+    onClick,
+  }: {
+    children: ReactNode;
+    onClick?: () => void;
+  }) => (
+    <button type="button" onClick={onClick} data-testid="dropdown-item">
+      {children}
+    </button>
+  ),
+}));
+
+import { ThemeToggle } from "./theme-toggle";
+
+function setTheme(t: string | undefined) {
+  useThemeReturn.theme = t;
+}
+
+describe("ThemeToggle — rendered markup", () => {
+  beforeEach(() => {
+    setThemeMock.mockReset();
+    setTheme("system");
+  });
+
+  it("shows the system label + monitor icon when theme is 'system'", () => {
+    setTheme("system");
+    const html = renderToStaticMarkup(<ThemeToggle />);
+    expect(html).toMatch(/lucide-monitor/);
+    expect(html).toContain(">system<");
+  });
+
+  it("shows the light label + sun icon when theme is 'light'", () => {
+    setTheme("light");
+    const html = renderToStaticMarkup(<ThemeToggle />);
+    expect(html).toMatch(/lucide-sun/);
+    expect(html).toContain(">light<");
+  });
+
+  it("shows the dark label + moon icon when theme is 'dark'", () => {
+    setTheme("dark");
+    const html = renderToStaticMarkup(<ThemeToggle />);
+    expect(html).toMatch(/lucide-moon/);
+    expect(html).toContain(">dark<");
+  });
+
+  it("falls back to 'system' label when next-themes returns undefined (pre-mount)", () => {
+    setTheme(undefined);
+    const html = renderToStaticMarkup(<ThemeToggle />);
+    expect(html).toContain(">system<");
+  });
+
+  it("renders three menu items (Light / Dark / System)", () => {
+    setTheme("system");
+    const html = renderToStaticMarkup(<ThemeToggle />);
+    expect(html).toContain("Light");
+    expect(html).toContain("Dark");
+    expect(html).toContain("System");
+    // One trigger + three menu items.
+    const items = html.match(/data-testid="dropdown-item"/g) ?? [];
+    expect(items).toHaveLength(3);
+  });
+});
+
+describe("ThemeToggle — onClick wires through to setTheme", () => {
+  beforeEach(() => {
+    setThemeMock.mockReset();
+    setTheme("system");
+  });
+
+  // We can't realistically click a server-rendered button. Instead reach
+  // into the rendered React tree and grab the onClick handlers — that's
+  // exactly what the click handler will fire in the browser, so it's a
+  // faithful contract check.
+  function getMenuItemHandlers(): Array<() => void> {
+    // Render via React.createElement to keep the element tree.
+    // Then walk it for the dropdown-item buttons.
+    // (Easier than parsing the SSR markup.)
+    const el = (<ThemeToggle />) as unknown as React.ReactElement;
+    const handlers: Array<() => void> = [];
+    function visit(node: unknown): void {
+      if (!node) return;
+      if (Array.isArray(node)) {
+        for (const c of node) visit(c);
+        return;
+      }
+      if (typeof node !== "object") return;
+      const n = node as { props?: Record<string, unknown>; type?: unknown };
+      const props = n.props ?? {};
+      if (typeof props.onClick === "function" && props["data-testid"] === "dropdown-item") {
+        handlers.push(props.onClick as () => void);
+      }
+      // type can be a function component — render it once with its props
+      // so we can recurse into its output. Component functions used here
+      // (DropdownMenuItem mocks etc.) are pure render-children.
+      if (typeof n.type === "function") {
+        const rendered = (n.type as (p: unknown) => unknown)(props);
+        visit(rendered);
+      }
+      if ("children" in props) {
+        visit(props.children);
+      }
+    }
+    visit(el);
+    return handlers;
+  }
+
+  it("first menu item sets theme to 'light'", () => {
+    const [light] = getMenuItemHandlers();
+    light?.();
+    expect(setThemeMock).toHaveBeenCalledTimes(1);
+    expect(setThemeMock).toHaveBeenCalledWith("light");
+  });
+
+  it("second menu item sets theme to 'dark'", () => {
+    const [, dark] = getMenuItemHandlers();
+    dark?.();
+    expect(setThemeMock).toHaveBeenCalledWith("dark");
+  });
+
+  it("third menu item sets theme to 'system'", () => {
+    const [, , system] = getMenuItemHandlers();
+    system?.();
+    expect(setThemeMock).toHaveBeenCalledWith("system");
+  });
+});