feat(confidence-engine): add dark mode

tests/theme-preference.test.js

@@ -0,0 +1,36 @@
+import { describe, expect, it } from "vitest";
+import {
+  readThemePreference,
+  resolveTheme,
+  saveThemePreference,
+  THEME_STORAGE_KEY,
+  toggleTheme,
+} from "@/lib/theme-preference.js";
+
+function makeStorage(initial = {}) {
+  const values = new Map(Object.entries(initial));
+  return {
+    getItem: (key) => values.get(key) ?? null,
+    setItem: (key, value) => values.set(key, value),
+  };
+}
+
+describe("theme preference", () => {
+  it("restores a saved dark preference ahead of system preference", () => {
+    const storage = makeStorage({ [THEME_STORAGE_KEY]: "dark" });
+    expect(resolveTheme({ savedTheme: readThemePreference(storage), systemPrefersDark: false })).toBe("dark");
+  });
+
+  it("uses system dark preference when no explicit choice exists", () => {
+    expect(resolveTheme({ savedTheme: null, systemPrefersDark: true })).toBe("dark");
+  });
+
+  it("switches themes and persists the explicit choice", () => {
+    const storage = makeStorage();
+    const nextTheme = toggleTheme("light");
+    saveThemePreference(nextTheme, storage);
+
+    expect(nextTheme).toBe("dark");
+    expect(readThemePreference(storage)).toBe("dark");
+  });
+});