添加项目文件。

Controllers/BaseController.cs

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+using AcdiuTools.Services;
+using Microsoft.AspNetCore.Mvc;
+using Microsoft.AspNetCore.Mvc.Filters;
+
+namespace AcdiuTools.Controllers
+{
+    /// <summary>
+    /// 基础控制器
+    /// 所有需要主题功能的控制器应继承此类
+    /// </summary>
+    public class BaseController(IThemeService themeService) : Controller
+    {
+        private readonly IThemeService _themeService = themeService;
+
+        /// <summary>
+        /// 在执行 Action 之前加载主题配置并注入 ViewBag
+        /// </summary>
+        public override async void OnActionExecuting(ActionExecutingContext context)
+        {
+            base.OnActionExecuting(context);
+
+            // 异步操作在 OnActionExecuting 中处理需小心，通常建议在 Action 内部或中间件中处理
+            // 这里为了演示简单，使用同步等待或改为中间件方案更佳
+            // 更好的方式：使用 ViewComponent 或 TagHelper
+
+            var theme = await _themeService.GetUserThemeAsync();
+            ViewBag.CurrentTheme = theme;
+        }
+    }
+}

Controllers/HomeController.cs

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+using AcdiuTools.Models;
+using AcdiuTools.Services;
+using Microsoft.AspNetCore.Mvc;
+using System.ComponentModel.DataAnnotations;
+using System.Diagnostics;
+using System.Security.Claims;
+
+namespace AcdiuTools.Controllers
+{
+    public class HomeController(IThemeService themeService) : BaseController(themeService)
+    {
+        private readonly IThemeService _themeService = themeService;
+
+        [Route("")]
+        public IActionResult Index()
+        {
+            return View();
+        }
+
+        [Route("privacy")]
+        public IActionResult Privacy()
+        {
+            return View();
+        }
+
+        [Route("err")]
+        [Route("error")]
+        [ResponseCache(Duration = 0, Location = ResponseCacheLocation.None, NoStore = true)]
+        public IActionResult Error()
+        {
+            return View(new ErrorViewModel { RequestId = Activity.Current?.Id ?? HttpContext.TraceIdentifier });
+        }
+
+        /// <summary>
+        /// API 端点：用于前端 AJAX 调用以切换主题
+        /// </summary>
+        [HttpPost]
+        public async Task<IActionResult> SetTheme([FromBody] ThemeRequest model)
+        {
+            // 获取用户 ID
+            string? claimUserId = User.FindFirst(ClaimTypes.NameIdentifier)?.Value
+                               ?? User.FindFirst("sub")?.Value;
+
+            string userId;
+
+            if (!string.IsNullOrEmpty(claimUserId))
+            {
+                // 已登录用户
+                userId = claimUserId;
+            }
+            else
+            {
+                // 匿名用户：尝试使用 Session ID
+                // 注意：访问 HttpContext.Session 会触发 Session 创建/加载
+                try
+                {
+                    userId = HttpContext.Session.Id;
+                }
+                catch (InvalidOperationException)
+                {
+                    // 如果 Session 未配置（防御性编程），回退到匿名标识
+                    userId = "anonymous_" + HttpContext.Connection.RemoteIpAddress?.ToString()?.Replace(".", "_") ?? "unknown";
+                }
+            }
+
+            await _themeService.SaveUserThemeAsync(userId, model.Theme);
+
+            return Json(new { success = true, theme = model.Theme });
+        }
+    }
+
+    /// <summary>
+    /// 主题请求模型
+    /// </summary>
+    public class ThemeRequest
+    {
+        /// <summary>
+        /// 主题名称
+        /// 修复 CS8618: 使用 required 修饰符，确保反序列化时必须存在该字段
+        /// </summary>
+        [Required]
+        public required string Theme { get; set; }
+    }
+}