diff --git a/frontend-web/src/App.tsx b/frontend-web/src/App.tsx
index 155c3f64..b7508f12 100644
--- a/frontend-web/src/App.tsx
+++ b/frontend-web/src/App.tsx
@@ -31,13 +31,15 @@ function AppProviders({ children }: { children: React.ReactNode }) {
       disableTransitionOnChange
       forcedTheme={forcedTheme}
     >
-      <I18nProvider initialLocale={locale}>
-        <ThemeProvider>
-          <ChatProvider>
-            <QueryClientProvider>{children}</QueryClientProvider>
-          </ChatProvider>
-        </ThemeProvider>
-      </I18nProvider>
+      {/* QueryClientProvider must wrap ThemeProvider because the custom
+          ThemeProvider reads the admin-configured default via useQuery. */}
+      <QueryClientProvider>
+        <I18nProvider initialLocale={locale}>
+          <ThemeProvider>
+            <ChatProvider>{children}</ChatProvider>
+          </ThemeProvider>
+        </I18nProvider>
+      </QueryClientProvider>
     </NextThemesProvider>
   );
 }
diff --git a/frontend-web/src/core/system-settings/api.ts b/frontend-web/src/core/system-settings/api.ts
index 697e88c5..e6335706 100644
--- a/frontend-web/src/core/system-settings/api.ts
+++ b/frontend-web/src/core/system-settings/api.ts
@@ -65,6 +65,48 @@ export async function loadPrivateSquareStatus(): Promise<PrivateSquareStatus> {
   return (await response.json()) as PrivateSquareStatus;
 }
 
+export type ThemeMode = "light" | "dark";
+
+export interface AppearanceSettings {
+  default_theme: ThemeMode;
+}
+
+export interface AppearanceUpdate {
+  default_theme?: ThemeMode;
+}
+
+export async function loadAppearanceSettings(): Promise<AppearanceSettings> {
+  const response = await apiFetch(
+    `${getBackendBaseURL()}/api/system-settings/appearance`,
+  );
+  if (!response.ok) {
+    throw new Error(
+      `Failed to load appearance settings: ${response.status}`,
+    );
+  }
+  return (await response.json()) as AppearanceSettings;
+}
+
+export async function updateAppearanceSettings(
+  body: AppearanceUpdate,
+): Promise<AppearanceSettings> {
+  const response = await apiFetch(
+    `${getBackendBaseURL()}/api/system-settings/appearance`,
+    {
+      method: "PUT",
+      headers: { "Content-Type": "application/json" },
+      body: JSON.stringify(body),
+    },
+  );
+  if (!response.ok) {
+    const detail = await response
+      .json()
+      .catch(() => ({ detail: "Failed to update appearance" }));
+    throw new Error(detail.detail ?? "Failed to update appearance");
+  }
+  return (await response.json()) as AppearanceSettings;
+}
+
 export async function updatePrivateSquareSettings(
   body: PrivateSquareUpdate,
 ): Promise<PrivateSquareStatus> {
diff --git a/frontend-web/src/core/system-settings/hooks.ts b/frontend-web/src/core/system-settings/hooks.ts
index 9c2ee5fd..903737a5 100644
--- a/frontend-web/src/core/system-settings/hooks.ts
+++ b/frontend-web/src/core/system-settings/hooks.ts
@@ -1,16 +1,20 @@
 import { useMutation, useQuery, useQueryClient } from "@tanstack/react-query";
 
 import {
+  loadAppearanceSettings,
   loadPrivateSquareStatus,
   loadPromptPrefixSettings,
+  updateAppearanceSettings,
   updatePrivateSquareSettings,
   updatePromptPrefixSettings,
+  type AppearanceUpdate,
   type PrivateSquareUpdate,
   type PromptPrefixUpdate,
 } from "./api";
 
 const PROMPT_PREFIX_KEY = ["systemSettings", "promptPrefix"] as const;
 const PRIVATE_SQUARE_KEY = ["systemSettings", "privateSquare"] as const;
+const APPEARANCE_KEY = ["systemSettings", "appearance"] as const;
 
 export function usePromptPrefixSettings() {
   return useQuery({
@@ -51,3 +55,25 @@ export function useUpdatePrivateSquareSettings() {
     },
   });
 }
+
+export function useAppearanceSettings() {
+  return useQuery({
+    queryKey: APPEARANCE_KEY,
+    queryFn: () => loadAppearanceSettings(),
+    staleTime: 60_000,
+    refetchOnWindowFocus: false,
+    // The provider runs even on the login page (where the user has no auth)
+    // — gracefully tolerate the fetch failing rather than retrying.
+    retry: false,
+  });
+}
+
+export function useUpdateAppearanceSettings() {
+  const queryClient = useQueryClient();
+  return useMutation({
+    mutationFn: (body: AppearanceUpdate) => updateAppearanceSettings(body),
+    onSuccess: (data) => {
+      queryClient.setQueryData(APPEARANCE_KEY, data);
+    },
+  });
+}
diff --git a/offline-backend-20260512/backend/app/gateway/routers/system_settings.py b/offline-backend-20260512/backend/app/gateway/routers/system_settings.py
index 48940387..aadb877e 100644
--- a/offline-backend-20260512/backend/app/gateway/routers/system_settings.py
+++ b/offline-backend-20260512/backend/app/gateway/routers/system_settings.py
@@ -3,6 +3,7 @@
 from __future__ import annotations
 
 import logging
+from typing import Literal
 
 from fastapi import APIRouter, HTTPException, Request
 from pydantic import BaseModel, Field
@@ -124,3 +125,42 @@ async def update_private_square(body: PrivateSquareUpdateRequest, request: Reque
         enabled=saved.private_square.enabled,
         password_set=bool(saved.private_square.password_hash),
     )
+
+
+class AppearanceResponse(BaseModel):
+    """Effective system-wide appearance defaults."""
+
+    default_theme: Literal["light", "dark"] = Field(..., description="System default theme — light or dark")
+
+
+class AppearanceUpdateRequest(BaseModel):
+    default_theme: Literal["light", "dark"] | None = Field(default=None)
+
+
+@router.get("/appearance", response_model=AppearanceResponse)
+async def get_appearance_settings() -> AppearanceResponse:
+    """Return the global default theme. Any logged-in user may read this."""
+    settings = load_system_settings().appearance
+    return AppearanceResponse(default_theme=settings.default_theme)
+
+
+@router.put("/appearance", response_model=AppearanceResponse)
+async def update_appearance_settings(
+    body: AppearanceUpdateRequest, request: Request
+) -> AppearanceResponse:
+    """Update the global appearance defaults. Admin only."""
+    await _require_admin(request)
+
+    current = load_system_settings()
+    fields = body.model_dump(exclude_unset=True)
+    update: dict[str, object] = {}
+    if "default_theme" in fields and fields["default_theme"] is not None:
+        update["default_theme"] = fields["default_theme"]
+    if update:
+        updated_appearance = current.appearance.model_copy(update=update)
+        saved = save_system_settings(
+            current.model_copy(update={"appearance": updated_appearance})
+        )
+    else:
+        saved = current
+    return AppearanceResponse(default_theme=saved.appearance.default_theme)
diff --git a/offline-backend-20260512/backend/packages/harness/deerflow/config/system_settings.py b/offline-backend-20260512/backend/packages/harness/deerflow/config/system_settings.py
index 0a2d41ea..67e1a4d9 100644
--- a/offline-backend-20260512/backend/packages/harness/deerflow/config/system_settings.py
+++ b/offline-backend-20260512/backend/packages/harness/deerflow/config/system_settings.py
@@ -17,6 +17,7 @@ import json
 import logging
 import threading
 from pathlib import Path
+from typing import Literal
 
 from pydantic import BaseModel, ConfigDict, Field
 
@@ -37,6 +38,22 @@ class PromptPrefixSettings(BaseModel):
     model_config = ConfigDict(extra="ignore")
 
 
+class AppearanceSettings(BaseModel):
+    """System-wide appearance defaults that any *new* user inherits.
+
+    Once a user explicitly toggles their theme in the UI, that choice is
+    persisted in the browser and supersedes this default — so changing
+    ``default_theme`` here only affects users who have never touched the
+    toggle, and any future first-time loads.
+    """
+
+    default_theme: Literal["light", "dark"] = Field(
+        default="light",
+        description="Theme applied for users who haven't picked their own (light / dark)",
+    )
+    model_config = ConfigDict(extra="ignore")
+
+
 class PrivateSquareSettings(BaseModel):
     """Admin-configurable settings for the agent private-square (私密广场).
 
@@ -56,6 +73,7 @@ class SystemSettings(BaseModel):
 
     prompt_prefix: PromptPrefixSettings = Field(default_factory=PromptPrefixSettings)
     private_square: PrivateSquareSettings = Field(default_factory=PrivateSquareSettings)
+    appearance: AppearanceSettings = Field(default_factory=AppearanceSettings)
     model_config = ConfigDict(extra="ignore")