diff --git a/src/main/themeMenuBuilder.js b/src/main/themeMenuBuilder.js
new file mode 100644
index 0000000..23011d8
--- /dev/null
+++ b/src/main/themeMenuBuilder.js
@@ -0,0 +1,50 @@
+/**
+ * Editor Theme menu builder — converts ThemeRegistry output into Electron
+ * MenuItem[] suitable for the View → Theme submenu.
+ *
+ * Pure: takes `setTheme` + `getCurrentThemeId` as injected dependencies so
+ * the builder never touches electron-store or `mainWindow`. Tested in
+ * isolation under `@jest-environment node`.
+ *
+ * Menu shape (matches the previous hardcoded block in src/main.js:1137-1245):
+ *   [item, item, …, { type: 'separator' }, item, item, …, separator, …]
+ * grouped by category in registry order, with separators between non-empty
+ * categories. Each theme item is a radio-style MenuItem so Electron shows a
+ * checkmark next to the active theme.
+ *
+ * @module themeMenuBuilder
+ */
+
+const ThemeRegistry = require('./ThemeRegistry');
+
+/**
+ * @param {object} deps
+ * @param {(id: string) => void} deps.setTheme
+ * @param {() => string} deps.getCurrentThemeId
+ * @returns {Array<object>} Electron MenuItemTemplate[]
+ */
+function buildThemeMenu({ setTheme, getCurrentThemeId }) {
+  const currentId = getCurrentThemeId();
+  const items = [];
+  const cats = ThemeRegistry.categories();
+  let nonEmptySeen = 0;
+
+  for (const cat of cats) {
+    const inCat = ThemeRegistry.list().filter((t) => t.category === cat);
+    if (inCat.length === 0) continue;
+    if (nonEmptySeen > 0) items.push({ type: 'separator' });
+    nonEmptySeen++;
+
+    for (const t of inCat) {
+      items.push({
+        label: t.label,
+        type: 'radio',
+        checked: t.id === currentId,
+        click: () => setTheme(t.id),
+      });
+    }
+  }
+  return items;
+}
+
+module.exports = { buildThemeMenu };
diff --git a/tests/main/theme-menu-builder.test.js b/tests/main/theme-menu-builder.test.js
new file mode 100644
index 0000000..2e0a0fc
--- /dev/null
+++ b/tests/main/theme-menu-builder.test.js
@@ -0,0 +1,76 @@
+/**
+ * @jest-environment node
+ *
+ * themeMenuBuilder tests — pure module. We pass in a fake `setTheme` and a
+ * fake `getCurrentThemeId` so we never touch electron-store or Electron.
+ *
+ * Note on isolation: ThemeRegistry holds state in a module-scoped Map.
+ * `jest.resetModules()` drops the cache so each test re-requires a fresh
+ * ThemeRegistry + buildThemeMenu pair, preventing leakage across tests.
+ */
+let ThemeRegistry;
+let buildThemeMenu;
+
+beforeEach(() => {
+  jest.resetModules();
+  ThemeRegistry = require('../../src/main/ThemeRegistry');
+  buildThemeMenu = require('../../src/main/themeMenuBuilder').buildThemeMenu;
+  ThemeRegistry.clear();
+});
+
+describe('buildThemeMenu', () => {
+  test('groups themes by category in registry order', () => {
+    ThemeRegistry.register({ id: 'l1', label: 'L1', category: 'light', isDark: false });
+    ThemeRegistry.register({ id: 'l2', label: 'L2', category: 'light', isDark: false });
+    ThemeRegistry.register({ id: 'd1', label: 'D1', category: 'dark', isDark: true });
+    ThemeRegistry.register({ id: 'd2', label: 'D2', category: 'dark', isDark: true });
+    ThemeRegistry.register({ id: 'hc1', label: 'HC1', category: 'high-contrast', isDark: true });
+    ThemeRegistry.register({ id: 's1', label: 'S1', category: 'seasonal', isDark: false });
+
+    const setTheme = jest.fn();
+    const items = buildThemeMenu({ setTheme, getCurrentThemeId: () => 'd1' });
+
+    // Expected order: light (2 items), separator, dark (2 items), separator, hc (1), separator, seasonal (1)
+    expect(items.map((i) => i.label || i.type)).toEqual([
+      'L1',
+      'L2',
+      'separator',
+      'D1',
+      'D2',
+      'separator',
+      'HC1',
+      'separator',
+      'S1',
+    ]);
+  });
+
+  test('marks the currently-selected theme as checked=true', () => {
+    ThemeRegistry.register({ id: 'a', label: 'A', category: 'light', isDark: false });
+    ThemeRegistry.register({ id: 'b', label: 'B', category: 'dark', isDark: true });
+
+    const items = buildThemeMenu({ setTheme: jest.fn(), getCurrentThemeId: () => 'b' });
+    const aItem = items.find((i) => i.label === 'A');
+    const bItem = items.find((i) => i.label === 'B');
+    expect(aItem.type).toBe('radio');
+    expect(aItem.checked).toBe(false);
+    expect(bItem.type).toBe('radio');
+    expect(bItem.checked).toBe(true);
+  });
+
+  test('click handler calls setTheme with the theme id', () => {
+    ThemeRegistry.register({ id: 'a', label: 'A', category: 'light', isDark: false });
+    const setTheme = jest.fn();
+    const items = buildThemeMenu({ setTheme, getCurrentThemeId: () => 'a' });
+    items[0].click();
+    expect(setTheme).toHaveBeenCalledWith('a');
+  });
+
+  test('falls back to no theme checked when current id unknown', () => {
+    ThemeRegistry.register({ id: 'a', label: 'A', category: 'light', isDark: false });
+    const items = buildThemeMenu({
+      setTheme: jest.fn(),
+      getCurrentThemeId: () => 'deleted-theme',
+    });
+    expect(items.find((i) => i.label === 'A').checked).toBe(false);
+  });
+});