Style ToolTip to make it dark enough in Dark mode

src/System.Windows.Forms/src/System/Windows/Forms/Controls/ListView/ListView.cs

@@ -10,6 +10,7 @@
 using System.Runtime.InteropServices;
 using System.Windows.Forms.Layout;
 using System.Windows.Forms.VisualStyles;
+using Windows.Win32.Graphics.Dwm;
 using Windows.Win32.System.Variant;
 using Windows.Win32.UI.Accessibility;
 using Windows.Win32.UI.Input.KeyboardAndMouse;
@@ -4512,7 +4513,7 @@ protected override void OnFontChanged(EventArgs e)
         InvalidateColumnHeaders();
     }
 
-    protected override void OnHandleCreated(EventArgs e)
+    protected override unsafe void OnHandleCreated(EventArgs e)
     {
         // don't persist flipViewToLargeIconAndSmallIcon across handle recreations...
         FlipViewToLargeIconAndSmallIcon = false;
@@ -4533,6 +4534,21 @@ protected override void OnHandleCreated(EventArgs e)
             // Get the ListView's ColumnHeader handle:
             HWND columnHeaderHandle = (HWND)PInvokeCore.SendMessage(this, PInvoke.LVM_GETHEADER, (WPARAM)0, (LPARAM)0);
             PInvoke.SetWindowTheme(columnHeaderHandle, $"{DarkModeIdentifier}_{ItemsViewThemeIdentifier}", null);
+
+            // Get the ListView's ToolTip handle:
+            HWND toolTipHandle = (HWND)PInvokeCore.SendMessage(this, PInvoke.LVM_GETTOOLTIPS, (WPARAM)0, (LPARAM)0);
+            PInvoke.SetWindowTheme(toolTipHandle, $"{DarkModeIdentifier}_{ExplorerThemeIdentifier}", null);
+
+            // Round the corners of the ToolTip window.
+            if (OsVersion.IsWindows11_OrGreater())
+            {
+                DWM_WINDOW_CORNER_PREFERENCE roundSmall = DWM_WINDOW_CORNER_PREFERENCE.DWMWCP_ROUNDSMALL;
+                PInvoke.DwmSetWindowAttribute(
+                    toolTipHandle,
+                    DWMWINDOWATTRIBUTE.DWMWA_WINDOW_CORNER_PREFERENCE,
+                    &roundSmall,
+                    sizeof(DWM_WINDOW_CORNER_PREFERENCE));
+            }
         }
 #pragma warning restore WFO5001
 

src/System.Windows.Forms/src/System/Windows/Forms/Controls/TabControl/TabControl.cs

@@ -7,6 +7,7 @@
 using System.Drawing.Design;
 using System.Text;
 using System.Windows.Forms.Layout;
+using Windows.Win32.Graphics.Dwm;
 using Windows.Win32.UI.Accessibility;
 
 namespace System.Windows.Forms;
@@ -1221,7 +1222,7 @@ protected override void OnGotFocus(EventArgs e)
     ///  We do some work here to configure the handle.
     ///  Overriders should call base.OnHandleCreated()
     /// </summary>
-    protected override void OnHandleCreated(EventArgs e)
+    protected override unsafe void OnHandleCreated(EventArgs e)
     {
         if (!IsHandleCreated)
         {
@@ -1259,6 +1260,24 @@ protected override void OnHandleCreated(EventArgs e)
                     HWND.HWND_TOPMOST,
                     0, 0, 0, 0,
                     SET_WINDOW_POS_FLAGS.SWP_NOMOVE | SET_WINDOW_POS_FLAGS.SWP_NOSIZE | SET_WINDOW_POS_FLAGS.SWP_NOACTIVATE);
+
+#pragma warning disable WFO5001
+                if (Application.IsDarkModeEnabled)
+                {
+                    PInvoke.SetWindowTheme(tooltipHwnd, $"{DarkModeIdentifier}_{ExplorerThemeIdentifier}", null);
+
+                    // Round the corners of the ToolTip window.
+                    if (OsVersion.IsWindows11_OrGreater())
+                    {
+                        DWM_WINDOW_CORNER_PREFERENCE roundSmall = DWM_WINDOW_CORNER_PREFERENCE.DWMWCP_ROUNDSMALL;
+                        PInvoke.DwmSetWindowAttribute(
+                            tooltipHwnd,
+                            DWMWINDOWATTRIBUTE.DWMWA_WINDOW_CORNER_PREFERENCE,
+                            &roundSmall,
+                            sizeof(DWM_WINDOW_CORNER_PREFERENCE));
+                    }
+                }
+#pragma warning restore WFO5001
             }
         }
 

src/System.Windows.Forms/src/System/Windows/Forms/Controls/TreeView/TreeView.cs

@@ -8,6 +8,7 @@
 using System.Runtime.InteropServices;
 using System.Windows.Forms.Layout;
 using System.Windows.Forms.VisualStyles;
+using Windows.Win32.Graphics.Dwm;
 using Windows.Win32.System.Variant;
 using Windows.Win32.UI.Accessibility;
 using static System.Windows.Forms.TreeNode;
@@ -1839,7 +1840,7 @@ protected override bool IsInputKey(Keys keyData)
     protected virtual void OnDrawNode(DrawTreeNodeEventArgs e) =>
         _onDrawNode?.Invoke(this, e);
 
-    protected override void OnHandleCreated(EventArgs e)
+    protected override unsafe void OnHandleCreated(EventArgs e)
     {
         if (!IsHandleCreated)
         {
@@ -1893,6 +1894,24 @@ protected override void OnHandleCreated(EventArgs e)
         {
             PInvokeCore.SendMessage(this, PInvoke.TVM_SETTEXTCOLOR, 0, c.ToWin32());
         }
+
+        if (Application.IsDarkModeEnabled)
+        {
+            // Get the TreeView's ToolTip handle:
+            HWND toolTipHandle = (HWND)PInvokeCore.SendMessage(HWND, PInvoke.TVM_GETTOOLTIPS, (WPARAM)0, (LPARAM)0);
+            PInvoke.SetWindowTheme(toolTipHandle, $"{DarkModeIdentifier}_{ExplorerThemeIdentifier}", null);
+
+            // Round the corners of the ToolTip window.
+            if (OsVersion.IsWindows11_OrGreater())
+            {
+                DWM_WINDOW_CORNER_PREFERENCE roundSmall = DWM_WINDOW_CORNER_PREFERENCE.DWMWCP_ROUNDSMALL;
+                PInvoke.DwmSetWindowAttribute(
+                    toolTipHandle,
+                    DWMWINDOWATTRIBUTE.DWMWA_WINDOW_CORNER_PREFERENCE,
+                    &roundSmall,
+                    sizeof(DWM_WINDOW_CORNER_PREFERENCE));
+            }
+        }
 #pragma warning restore WFO5001
 
         // Put the LineColor into the native control only if set.

src/System.Windows.Forms/src/System/Windows/Forms/ToolTip/ToolTip.cs

@@ -3,6 +3,7 @@
 
 using System.ComponentModel;
 using System.Drawing;
+using Windows.Win32.Graphics.Dwm;
 
 namespace System.Windows.Forms;
 
@@ -714,10 +715,30 @@ private unsafe void CreateHandle()
 
             _window.CreateHandle(cp);
 
+#pragma warning disable WFO5001
             if (SystemInformation.HighContrast)
             {
                 PInvoke.SetWindowTheme(HWND, string.Empty, string.Empty);
             }
+            else if (Application.IsDarkModeEnabled)
+            {
+                if (!_isBalloon && OsVersion.IsWindows11_OrGreater())
+                {
+                    DWM_WINDOW_CORNER_PREFERENCE roundSmall = DWM_WINDOW_CORNER_PREFERENCE.DWMWCP_ROUNDSMALL;
+                    PInvoke.DwmSetWindowAttribute(
+                        HWND,
+                        DWMWINDOWATTRIBUTE.DWMWA_WINDOW_CORNER_PREFERENCE,
+                        &roundSmall,
+                        sizeof(DWM_WINDOW_CORNER_PREFERENCE));
+                }
+
+                PInvokeCore.SendMessage(
+                   HWND,
+                   PInvoke.TTM_SETWINDOWTHEME,
+                   default,
+                   $"{Control.DarkModeIdentifier}_{Control.ExplorerThemeIdentifier}");
+            }
+#pragma warning restore WFO5001
         }
 
         // If in OwnerDraw mode, we don't want the default border.

src/System.Windows.Forms/tests/TestUtilities/ApplicationColorModeScope.cs

@@ -0,0 +1,34 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Windows.Forms;
+
+namespace System;
+
+#pragma warning disable WFO5001
+/// <summary>
+///  Scope for setting the default color mode (dark mode) for the application. Use in a <see langword="using"/> statement.
+/// </summary>
+public readonly ref struct ApplicationColorModeScope
+{
+    private readonly SystemColorMode _originalColorMode;
+
+    public ApplicationColorModeScope(SystemColorMode colorMode)
+    {
+        _originalColorMode = Application.ColorMode;
+
+        if (_originalColorMode != colorMode)
+        {
+            Application.SetColorMode(colorMode);
+        }
+    }
+
+    public void Dispose()
+    {
+        if (Application.ColorMode != _originalColorMode)
+        {
+            Application.SetColorMode(_originalColorMode);
+        }
+    }
+}
+#pragma warning restore WFO5001

src/System.Windows.Forms/tests/UnitTests/System/Windows/Forms/ToolTipTests.cs

@@ -6,6 +6,7 @@
 using System.Windows.Forms.Automation;
 using Moq;
 using Moq.Protected;
+using Windows.Win32.Graphics.Dwm;
 
 namespace System.Windows.Forms.Tests;
 
@@ -782,6 +783,44 @@ public void ToolTip_ToString_Invoke_ReturnsExpected()
         Assert.Equal("System.Windows.Forms.ToolTip InitialDelay: 500, ShowAlways: False", toolTip.ToString());
     }
 
+#pragma warning disable WFO5001
+    [WinFormsTheory]
+    [BoolData]
+    public unsafe void ToolTip_DarkMode_GetCornerPreference_ReturnsExpected(bool value)
+    {
+        if (!OsVersion.IsWindows11_OrGreater())
+        {
+            return;
+        }
+
+        if (SystemInformation.HighContrast)
+        {
+            // We don't run this test in HighContrast mode.
+            return;
+        }
+
+        using ApplicationColorModeScope colorModeScope = new(colorMode: SystemColorMode.Dark);
+        using SubToolTip toolTip = new()
+        {
+            IsBalloon = value,
+        };
+
+        toolTip.Handle.Should().NotBe(IntPtr.Zero); // A workaround to create the toolTip native window Handle
+
+        DWM_WINDOW_CORNER_PREFERENCE cornerPreference;
+        PInvoke.DwmGetWindowAttribute(
+            toolTip.HWND,
+            DWMWINDOWATTRIBUTE.DWMWA_WINDOW_CORNER_PREFERENCE,
+            &cornerPreference,
+            sizeof(DWM_WINDOW_CORNER_PREFERENCE));
+
+        cornerPreference.Should().Be(
+            toolTip.IsBalloon
+                ? DWM_WINDOW_CORNER_PREFERENCE.DWMWCP_DEFAULT
+                : DWM_WINDOW_CORNER_PREFERENCE.DWMWCP_ROUNDSMALL);
+    }
+#pragma warning restore WFO5001
+
     [WinFormsFact]
     public void ToolTip_SetToolTipToControl_Invokes_SetToolTip_OfControl()
     {