默认主题改为浅色 + 管理员可配置全局默认主题

- system_settings 新增 AppearanceSettings(default_theme)，默认为 light，
  对应 GET /api/system-settings/appearance（任何人可读）+ PUT（admin 可写）
- App.tsx 把 NextThemesProvider 的 defaultTheme 从 dark 改为 light，并把
  QueryClientProvider 提到 ThemeProvider 外层以便后者使用 useQuery
- ThemeContext 新增 localStorage 标志 strategy-theme-user-set：用户调用
  toggleTheme / setTheme 时打标；每次启动若标志未设，则跟随 admin 配置的
  default_theme 同步主题，admin 改动用户刷新即生效；标志一旦存在永远尊重
  用户个人选择
- legacy 'theme' 键迁移视为用户显式选择，避免老用户被新默认覆盖
- frontend system-settings api/hook 增 appearance 拉取与更新

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

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)

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")