Add dark mode toggle with ThemeState service

Implement dark mode support using a new ThemeState service that manages theme switching and integrates with DevExpress Blazor theming. Update App.razor to apply the theme globally, enhance MainLayout with a toggle button and dynamic CSS classes, and add dark mode styles to CSS files. Register ThemeState as a scoped service in Program.cs.

DbFirst.BlazorWebApp/Services/ThemeState.cs

@@ -0,0 +1,35 @@
+using DevExpress.Blazor;
+
+namespace DbFirst.BlazorWebApp.Services;
+
+public class ThemeState
+{
+    private readonly IThemeChangeService themeChangeService;
+
+    public ThemeState(IThemeChangeService themeChangeService)
+    {
+        this.themeChangeService = themeChangeService;
+    }
+
+    public bool IsDarkMode { get; private set; }
+
+    public event Action? OnChange;
+
+    public void SetDarkMode(bool isDarkMode)
+    {
+        if (IsDarkMode == isDarkMode)
+        {
+            return;
+        }
+
+        IsDarkMode = isDarkMode;
+        var theme = Themes.Fluent.Clone(properties =>
+        {
+            properties.Mode = isDarkMode ? ThemeMode.Dark : ThemeMode.Light;
+            properties.ApplyToPageElements = true;
+        });
+
+        themeChangeService.SetTheme(theme);
+        OnChange?.Invoke();
+    }
+}