Add Windows dark-mode theming to downloader UI

Ra3.BattleNet.Downloader/MainWindow.xaml.cs

@@ -1,4 +1,12 @@
-using System.Windows;
+using System;
+using System.Diagnostics;
+using System.IO;
+using System.Security;
+using System.Runtime.InteropServices;
+using System.Windows;
+using System.Windows.Interop;
+using System.Windows.Media;
+using Microsoft.Win32;
 
 namespace Ra3.BattleNet.Downloader;
 
@@ -7,18 +15,34 @@ namespace Ra3.BattleNet.Downloader;
 /// </summary>
 public partial class MainWindow : Window
 {
+    private const string PersonalizeRegistryKeyPath = @"Software\Microsoft\Windows\CurrentVersion\Themes\Personalize";
+    private const int DarkModeRegistryValue = 0; // 0 indicates dark mode is enabled (AppsUseLightTheme registry value is 0)
+    private static readonly Brush DarkBackgroundBrush = new SolidColorBrush(Color.FromRgb(0x19, 0x19, 0x19));
+    private static readonly Brush DarkForegroundBrush = Brushes.White;
+
     public MainWindow()
     {
         InitializeComponent();
+        ApplySystemTheme();
         ChangeLanguage(chinese: true);
     }
 
+    protected override void OnSourceInitialized(EventArgs e)
+    {
+        base.OnSourceInitialized(e);
+        if (IsSystemInDarkMode())
+        {
+            TrySetDarkTitleBar();
+        }
+    }
+
     private async void Window_Loaded(object sender, RoutedEventArgs e)
     {
         await Download.BitTorrentDownload((ViewModel)DataContext);
     }
 
     private void ChineseCheckbox_Checked(object sender, RoutedEventArgs e) => ChangeLanguage(chinese: true);
+
     private void EnglishCheckbox_Checked(object sender, RoutedEventArgs e) => ChangeLanguage(chinese: false);
 
     private void ChangeLanguage(bool chinese)
@@ -35,4 +59,119 @@ private void Hyperlink_Click(object sender, RoutedEventArgs e)
     {
         Download.OpenAndLocateFile(Download.DownloadPath);
     }
-}
+
+    private void ApplySystemTheme()
+    {
+        if (!IsSystemInDarkMode())
+        {
+            return;
+        }
+
+        try
+        {
+            Background = DarkBackgroundBrush;
+            Foreground = DarkForegroundBrush;
+            DownloadProgressBar.Background = DarkBackgroundBrush;
+            ChineseCheckbox.Foreground = DarkForegroundBrush;
+            EnglishCheckbox.Foreground = DarkForegroundBrush;
+        }
+        catch
+        {
+            // If dark mode application fails, just continue with default colors
+        }
+    }
+
+    private static bool IsSystemInDarkMode()
+    {
+        try
+        {
+            using var personalize = Registry.CurrentUser.OpenSubKey(PersonalizeRegistryKeyPath);
+            if (personalize is null)
+            {
+                return false;
+            }
+
+            var appsUseLightTheme = personalize.GetValue("AppsUseLightTheme");
+            return appsUseLightTheme is int value && value == DarkModeRegistryValue;
+        }
+        catch (Exception ex) when (ex is SecurityException
+                                   or UnauthorizedAccessException
+                                   or ArgumentException
+                                   or ObjectDisposedException
+                                   or IOException)
+        {
+#if DEBUG
+            Debug.WriteLine($"Failed to read dark mode registry setting from {PersonalizeRegistryKeyPath}, falling back to light mode: {ex}");
+#endif
+            return false;
+        }
+    }
+
+    // Enable dark title bar (non-client area) for Windows 10 1809+ when dark mode is active.
+    // Returns true if dark title bar was successfully applied, false otherwise.
+    private bool TrySetDarkTitleBar()
+    {
+        try
+        {
+            var handle = new WindowInteropHelper(this).Handle;
+            if (handle == IntPtr.Zero)
+            {
+                return false;
+            }
+
+            // Get real Windows build from registry (Environment.OSVersion.Build may return compatibility version)
+            int build = GetWindowsBuild();
+            if (build == -1)
+            {
+                return false;
+            }
+
+            // Detect Windows build: 17763 (1809) supports attribute 19, 18362+ (1903) supports 20
+            var attribute = build >= 18362 ? DWMWA_USE_IMMERSIVE_DARK_MODE : (build >= 17763 ? DWMWA_USE_IMMERSIVE_DARK_MODE_BEFORE_1903 : -1);
+            if (attribute == -1)
+            {
+                return false;
+            }
+
+            int useDark = 1;
+            int result = DwmSetWindowAttribute(handle, attribute, ref useDark, sizeof(int));
+
+            // S_OK = 0, anything else is failure
+            return result == 0;
+        }
+        catch
+        {
+            return false;
+        }
+    }
+
+    private static int GetWindowsBuild()
+    {
+        try
+        {
+            using var key = Registry.LocalMachine.OpenSubKey(@"SOFTWARE\Microsoft\Windows NT\CurrentVersion");
+            if (key is null)
+            {
+                return -1;
+            }
+
+            var buildValue = key.GetValue("CurrentBuildNumber");
+            if (buildValue is string buildStr && int.TryParse(buildStr, out int build))
+            {
+                return build;
+            }
+
+            return -1;
+        }
+        catch
+        {
+            return -1;
+        }
+    }
+
+    private const int DWMWA_USE_IMMERSIVE_DARK_MODE = 20;
+    private const int DWMWA_USE_IMMERSIVE_DARK_MODE_BEFORE_1903 = 19;
+
+    [DllImport("dwmapi.dll")]
+    private static extern int DwmSetWindowAttribute(IntPtr hwnd, int dwAttribute, ref int pvAttribute, int cbAttribute);
+}