✨ feat: auto system theme

apps/web/app/editor/[projectId]/page.tsx

@@ -4,7 +4,7 @@ import React from "react";
 import { useParams } from "next/navigation";
 import { MousePointer2, Hand, ScanFace, Type, ImagePlus } from "lucide-react";
 import { useEditorStore } from "../../../store/editor-store";
-import { useUiStore } from "../../../store/ui-store";
+import { useUiStore, isDarkTheme } from "../../../store/ui-store";
 import { CanvasSizeModal } from "../../../components/canvas-size-modal";
 import { EditorCanvasStage } from "../../../components/editor/editor-canvas-stage";
 import { EditorHeader } from "../../../components/editor/editor-header";
@@ -148,7 +148,7 @@ export default function EditorPage() {
     t,
   });
 
-  const isDark = theme === "dark";
+  const isDark = isDarkTheme(theme);
   const { width: cw, height: ch } = project.canvas;
 
   const handleSave = React.useCallback(async () => {

apps/web/app/page.tsx

@@ -6,7 +6,7 @@ import { addLayer, createLayer, createProject, parseProjectFile, setCanvasSize }
 import type { Project } from "@pien-studio/types";
 import { deleteProject, duplicateProject, loadProjects, upsertProject } from "@pien-studio/storage";
 import { useEditorStore } from "../store/editor-store";
-import { useUiStore } from "../store/ui-store";
+import { useUiStore, isDarkTheme } from "../store/ui-store";
 import { accentButtonClass, cx, mutedSurfaceClass, subtleButtonClass, surfaceClass } from "../lib/theme";
 import { UiPreferences } from "../components/ui-preferences";
 import { useAssetCleanupJob } from "../hooks/use-asset-cleanup-job";
@@ -23,7 +23,7 @@ export default function HomePage() {
   const [projectPendingDelete, setProjectPendingDelete] = React.useState<Project | null>(null);
   const projectInputRef = React.useRef<HTMLInputElement | null>(null);
   const imageInputRef = React.useRef<HTMLInputElement | null>(null);
-  const isDark = theme === "dark";
+  const isDark = isDarkTheme(theme);
 
   const refreshProjects = React.useCallback(async () => {
     setProjects(await loadProjects());

apps/web/components/ui-preferences.tsx

@@ -2,13 +2,14 @@
 
 import React from "react";
 import { useUiStore } from "../store/ui-store";
+import { isDarkTheme } from "../store/ui-store";
 import { cx, subtleButtonClass } from "../lib/theme";
 import { useTranslations } from "../hooks/use-translations";
 
 export function UiPreferences({ compact = false }: { compact?: boolean }) {
   const { theme, locale, setTheme, setLocale } = useUiStore((s) => s);
   const { t } = useTranslations();
-  const isDark = theme === "dark";
+  const isDark = isDarkTheme(theme);
   const wrapperHeight = compact ? "h-7" : "h-9";
   const textSize = compact ? "text-[10px]" : "text-xs";
 
@@ -33,10 +34,13 @@ export function UiPreferences({ compact = false }: { compact?: boolean }) {
       </label>
       <button
         type="button"
-        onClick={() => setTheme(theme === "dark" ? "light" : "dark")}
+        onClick={() => {
+          const next = theme === "light" ? "dark" : theme === "dark" ? "system" : "light";
+          setTheme(next);
+        }}
         className={cx("rounded border px-3 font-semibold", subtleButtonClass(isDark), wrapperHeight, textSize)}
       >
-        {theme === "dark" ? t("ui.darkMode") : t("ui.lightMode")}
+        {theme === "dark" ? t("ui.darkMode") : theme === "system" ? t("ui.systemMode") : t("ui.lightMode")}
       </button>
     </div>
   );

apps/web/store/ui-store.ts

@@ -2,16 +2,22 @@
 
 import { create } from "zustand";
 
-export type AppTheme = "light" | "dark";
+export type AppTheme = "light" | "dark" | "system";
 export type AppLocale = "en" | "th" | "ja";
 
 const THEME_KEY = "pien.ui.theme";
 const LOCALE_KEY = "pien.ui.locale";
 
+function getSystemTheme(): "light" | "dark" {
+  if (typeof window === "undefined") return "light";
+  return window.matchMedia("(prefers-color-scheme: dark)").matches ? "dark" : "light";
+}
+
 function readTheme(): AppTheme {
   if (typeof window === "undefined") return "light";
   const value = window.localStorage.getItem(THEME_KEY);
-  return value === "light" ? "light" : "dark";
+  if (value === "light" || value === "dark" || value === "system") return value;
+  return "system";
 }
 
 function readLocale(): AppLocale {
@@ -23,7 +29,12 @@ function readLocale(): AppLocale {
 
 function applyTheme(theme: AppTheme): void {
   if (typeof document === "undefined") return;
-  document.documentElement.setAttribute("data-theme", theme);
+  const resolved = theme === "system" ? getSystemTheme() : theme;
+  document.documentElement.setAttribute("data-theme", resolved);
+}
+
+export function isDarkTheme(theme: AppTheme): boolean {
+  return theme === "system" ? getSystemTheme() === "dark" : theme === "dark";
 }
 
 type UiState = {