Style ToolTip to make it dark enough in Dark mode

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

@@ -714,10 +714,12 @@ private unsafe void CreateHandle()
 
             _window.CreateHandle(cp);
 
-            if (SystemInformation.HighContrast)
+#pragma warning disable WFO5001
+            if (SystemInformation.HighContrast || Application.IsDarkModeEnabled)
             {
                 PInvoke.SetWindowTheme(HWND, string.Empty, string.Empty);
             }
+#pragma warning restore WFO5001
         }
 
         // If in OwnerDraw mode, we don't want the default border.
@@ -770,15 +772,11 @@ private unsafe void CreateHandle()
         // Set active status.
         PInvokeCore.SendMessage(this, PInvoke.TTM_ACTIVATE, (WPARAM)(BOOL)_active);
 
-        if (BackColor != SystemColors.Info)
-        {
-            PInvokeCore.SendMessage(this, PInvoke.TTM_SETTIPBKCOLOR, (WPARAM)BackColor);
-        }
+        // Set background color.
+        PInvokeCore.SendMessage(this, PInvoke.TTM_SETTIPBKCOLOR, (WPARAM)_backColor);
 
-        if (ForeColor != SystemColors.InfoText)
-        {
-            PInvokeCore.SendMessage(this, PInvoke.TTM_SETTIPTEXTCOLOR, (WPARAM)ForeColor);
-        }
+        // Set text color.
+        PInvokeCore.SendMessage(this, PInvoke.TTM_SETTIPTEXTCOLOR, (WPARAM)_foreColor);
 
         if (_toolTipIcon > 0 || !string.IsNullOrEmpty(_toolTipTitle))
         {

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

@@ -0,0 +1,44 @@
+// 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)
+    {
+        // Prevent multiple ApplicationColorModeScopes from running simultaneously.
+        // Using Monitor to allow recursion on the same thread.
+        Monitor.Enter(typeof(ApplicationColorModeScope));
+        _originalColorMode = Application.ColorMode;
+
+        if (_originalColorMode != colorMode)
+        {
+            Application.SetColorMode(colorMode);
+        }
+    }
+
+    public void Dispose()
+    {
+        try
+        {
+            if (Application.ColorMode != _originalColorMode)
+            {
+                Application.SetColorMode(_originalColorMode);
+            }
+        }
+        finally
+        {
+            Monitor.Exit(typeof(ApplicationColorModeScope));
+        }
+    }
+}
+#pragma warning restore WFO5001

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

@@ -782,6 +782,32 @@ public void ToolTip_ToString_Invoke_ReturnsExpected()
         Assert.Equal("System.Windows.Forms.ToolTip InitialDelay: 500, ShowAlways: False", toolTip.ToString());
     }
 
+#pragma warning disable WFO5001
+    [WinFormsFact]
+    public void ToolTip_DarkMode_GetColors_ReturnsExpected()
+    {
+        if (SystemInformation.HighContrast)
+        {
+            // We don't run this test in HighContrast mode.
+            return;
+        }
+
+        using ApplicationColorModeScope colorModeScope = new(colorMode: SystemColorMode.Dark);
+        using SubToolTip toolTip = new();
+
+        toolTip.Handle.Should().NotBe(IntPtr.Zero); // A workaround to create the toolTip native window Handle
+
+        var backgroundColor = PInvokeCore.SendMessage(toolTip, PInvoke.TTM_GETTIPBKCOLOR);
+        Color backColor = ColorTranslator.FromWin32((int)backgroundColor);
+
+        var textColor = PInvokeCore.SendMessage(toolTip, PInvoke.TTM_GETTIPTEXTCOLOR);
+        Color foreColor = ColorTranslator.FromWin32((int)textColor);
+
+        backColor.ToArgb().Should().Be(SystemColors.Info.ToArgb());
+        foreColor.ToArgb().Should().Be(SystemColors.InfoText.ToArgb());
+    }
+#pragma warning restore WFO5001
+
     [WinFormsFact]
     public void ToolTip_SetToolTipToControl_Invokes_SetToolTip_OfControl()
     {