fix: respect system dark mode setting

Dalamud/Interface/Internal/InterfaceManager.cs

@@ -32,6 +32,7 @@ using Dalamud.Interface.Windowing;
 using Dalamud.Interface.Windowing.Persistence;
 using Dalamud.IoC.Internal;
 using Dalamud.Logging.Internal;
+using Dalamud.Memory;
 using Dalamud.Plugin.Services;
 using Dalamud.Utility;
 using Dalamud.Utility.Timing;
@@ -500,6 +501,34 @@ internal partial class InterfaceManager : IInternalDisposableService
         ImGuiHelpers.ClearStacksOnContext();
     }
 
+    /// <summary>
+    /// Applies immersive dark mode to the game window based on the current system theme setting.
+    /// </summary>
+    internal void SetImmersiveModeFromSystemTheme()
+    {
+        bool useDark = this.IsSystemInDarkMode();
+        this.SetImmersiveMode(useDark);
+    }
+
+    /// <summary>
+    /// Checks whether the system use dark mode.
+    /// </summary>
+    /// <returns>Returns true if dark mode is preferred.</returns>
+    internal bool IsSystemInDarkMode()
+    {
+        try
+        {
+            using var key = Microsoft.Win32.Registry.CurrentUser.OpenSubKey(
+                @"Software\Microsoft\Windows\CurrentVersion\Themes\Personalize");
+            var value = key?.GetValue("AppsUseLightTheme") as int?;
+            return value != 1;
+        }
+        catch
+        {
+            return false;
+        }
+    }
+
     /// <summary>
     /// Toggle Windows 11 immersive mode on the game window.
     /// </summary>
@@ -744,6 +773,18 @@ internal partial class InterfaceManager : IInternalDisposableService
 
     private void WndProcHookManagerOnPreWndProc(WndProcEventArgs args)
     {
+        if (args.Message == WM.WM_SETTINGCHANGE)
+        {
+            if (this.dalamudConfiguration.WindowIsImmersive)
+            {
+                if (MemoryHelper.EqualsZeroTerminatedWideString("ImmersiveColorSet", args.LParam) ||
+                    MemoryHelper.EqualsZeroTerminatedWideString("VisualStyleChanged", args.LParam))
+                {
+                    this.SetImmersiveModeFromSystemTheme();
+                }
+            }
+        }
+
         var r = this.backend?.ProcessWndProcW(args.Hwnd, args.Message, args.WParam, args.LParam);
         if (r is not null)
             args.SuppressWithValue(r.Value);
@@ -858,7 +899,7 @@ internal partial class InterfaceManager : IInternalDisposableService
         {
             // Requires that game window to be there, which will be the case once game swap chain is initialized.
             if (Service<DalamudConfiguration>.Get().WindowIsImmersive)
-                this.SetImmersiveMode(true);
+                this.SetImmersiveModeFromSystemTheme();
         }
         catch (Exception ex)
         {

Dalamud/Interface/Internal/Windows/Settings/Tabs/SettingsTabLook.cs

@@ -65,7 +65,14 @@ internal sealed class SettingsTabLook : SettingsTab
             {
                 try
                 {
-                    Service<InterfaceManager>.GetNullable()?.SetImmersiveMode(b);
+                    if (b)
+                    {
+                        Service<InterfaceManager>.GetNullable()?.SetImmersiveModeFromSystemTheme();
+                    }
+                    else
+                    {
+                        Service<InterfaceManager>.GetNullable()?.SetImmersiveMode(false);
+                    }
                 }
                 catch (Exception ex)
                 {

Dalamud/Memory/MemoryHelper.cs

@@ -264,6 +264,31 @@ public static unsafe class MemoryHelper
         }
     }
 
+    /// <summary>
+    /// Compares a UTF-16 character span with a null-terminated UTF-16 string at <paramref name="memoryAddress"/>.
+    /// </summary>
+    /// <param name="charSpan">UTF-16 character span (e.g., from a string literal).</param>
+    /// <param name="memoryAddress">Address of null-terminated UTF-16 (wide) string, as used by Windows APIs.</param>
+    /// <returns><see langword="true"/> if equal; otherwise, <see langword="false"/>.</returns>
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    public static unsafe bool EqualsZeroTerminatedWideString(
+        scoped ReadOnlySpan<char> charSpan,
+        nint memoryAddress)
+    {
+        if (memoryAddress == 0)
+            return charSpan.Length == 0;
+
+        char* p = (char*)memoryAddress;
+
+        foreach (char c in charSpan)
+        {
+            if (*p++ != c)
+                return false;
+        }
+
+        return *p == '\0';
+    }
+
     /// <summary>
     /// Read a UTF-8 encoded string from a specified memory address.
     /// </summary>