✨ 添加主题切换功能：支持暗色/亮色玻璃主题

功能说明：
- 管理员可在系统设置中配置全局默认主题
- 普通用户可在设置页面选择：跟随全局/暗色/亮色
- 分享页面自动继承分享者的主题偏好
- 主题设置实时保存，刷新后保持

技术实现：
- 后端：数据库添加theme_preference字段，新增主题API
- 前端：CSS变量实现主题切换，localStorage缓存避免闪烁
- 分享页：加载时获取分享者主题设置

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

backend/server.js

@@ -1628,6 +1628,56 @@ app.get('/api/user/profile', authMiddleware, (req, res) => {
   });
 });
 
+// 获取用户主题偏好（包含全局默认主题）
+app.get('/api/user/theme', authMiddleware, (req, res) => {
+  try {
+    const globalTheme = SettingsDB.get('global_theme') || 'dark';
+    const userTheme = req.user.theme_preference; // null表示跟随全局
+
+    res.json({
+      success: true,
+      theme: {
+        global: globalTheme,
+        user: userTheme,
+        effective: userTheme || globalTheme // 用户设置优先，否则使用全局
+      }
+    });
+  } catch (error) {
+    res.status(500).json({ success: false, message: '获取主题失败' });
+  }
+});
+
+// 设置用户主题偏好
+app.post('/api/user/theme', authMiddleware, (req, res) => {
+  try {
+    const { theme } = req.body;
+    const validThemes = ['dark', 'light', null]; // null表示跟随全局
+
+    if (!validThemes.includes(theme)) {
+      return res.status(400).json({
+        success: false,
+        message: '无效的主题设置，可选: dark, light, null(跟随全局)'
+      });
+    }
+
+    UserDB.update(req.user.id, { theme_preference: theme });
+
+    const globalTheme = SettingsDB.get('global_theme') || 'dark';
+    res.json({
+      success: true,
+      message: '主题偏好已更新',
+      theme: {
+        global: globalTheme,
+        user: theme,
+        effective: theme || globalTheme
+      }
+    });
+  } catch (error) {
+    console.error('更新主题失败:', error);
+    res.status(500).json({ success: false, message: '更新主题失败' });
+  }
+});
+
 // 更新FTP配置
 app.post('/api/user/update-ftp',
   authMiddleware,
@@ -2888,6 +2938,44 @@ app.delete('/api/share/:id', authMiddleware, (req, res) => {
 
 // ===== 分享链接访问（公开） =====
 
+// 获取公共主题设置（用于分享页面，无需认证）
+app.get('/api/public/theme', (req, res) => {
+  try {
+    const globalTheme = SettingsDB.get('global_theme') || 'dark';
+    res.json({
+      success: true,
+      theme: globalTheme
+    });
+  } catch (error) {
+    res.json({ success: true, theme: 'dark' }); // 出错默认暗色
+  }
+});
+
+// 获取分享页面主题（基于分享者偏好或全局设置）
+app.get('/api/share/:code/theme', (req, res) => {
+  try {
+    const { code } = req.params;
+    const share = ShareDB.findByCode(code);
+    const globalTheme = SettingsDB.get('global_theme') || 'dark';
+
+    if (!share) {
+      return res.json({
+        success: true,
+        theme: globalTheme
+      });
+    }
+
+    // 优先使用分享者的主题偏好，否则使用全局主题
+    const effectiveTheme = share.theme_preference || globalTheme;
+    res.json({
+      success: true,
+      theme: effectiveTheme
+    });
+  } catch (error) {
+    res.json({ success: true, theme: 'dark' });
+  }
+});
+
 // 访问分享链接 - 验证密码（支持本地存储和SFTP）
 app.post('/api/share/:code/verify', shareRateLimitMiddleware, async (req, res) => {
   const { code } = req.params;
@@ -3418,11 +3506,13 @@ app.get('/api/admin/settings', authMiddleware, adminMiddleware, (req, res) => {
     const smtpUser = SettingsDB.get('smtp_user');
     const smtpFrom = SettingsDB.get('smtp_from') || smtpUser;
     const smtpHasPassword = !!SettingsDB.get('smtp_password');
+    const globalTheme = SettingsDB.get('global_theme') || 'dark';
 
     res.json({
       success: true,
       settings: {
         max_upload_size: maxUploadSize,
+        global_theme: globalTheme,
         smtp: {
           host: smtpHost || '',
           port: smtpPort ? parseInt(smtpPort, 10) : 465,
@@ -3445,7 +3535,7 @@ app.get('/api/admin/settings', authMiddleware, adminMiddleware, (req, res) => {
 // 更新系统设置
 app.post('/api/admin/settings', authMiddleware, adminMiddleware, (req, res) => {
   try {
-    const { max_upload_size, smtp } = req.body;
+    const { max_upload_size, smtp, global_theme } = req.body;
 
     if (max_upload_size !== undefined) {
       const size = parseInt(max_upload_size);
@@ -3458,6 +3548,18 @@ app.post('/api/admin/settings', authMiddleware, adminMiddleware, (req, res) => {
       SettingsDB.set('max_upload_size', size.toString());
     }
 
+    // 更新全局主题
+    if (global_theme !== undefined) {
+      const validThemes = ['dark', 'light'];
+      if (!validThemes.includes(global_theme)) {
+        return res.status(400).json({
+          success: false,
+          message: '无效的主题设置'
+        });
+      }
+      SettingsDB.set('global_theme', global_theme);
+    }
+
     if (smtp) {
       if (!smtp.host || !smtp.port || !smtp.user) {
         return res.status(400).json({ success: false, message: 'SMTP配置不完整' });