[Dark Mode] ToolTip is not in dark mode after enabled SystemColorMode.Dark

[Olina-Zhang]

.NET version

9.0.0-rc.1.24418.2 + private dlls built from winforms repo main branch

Did it work in .NET Framework?

No

Did it work in any of the earlier releases of .NET Core or .NET 5+?

This is a new feature Dark Mode: #11857

Issue description

ToolTip is not in dark mode after enabled SystemColorMode.Dark

Steps to reproduce

1. Create a WinForms .NET application with checkBox and toolTip controls
2. Find the ToolTip on toolTip1 property of checkBox, then set some text for this property
3. Enable DarkMode by Application.SetColorMode(SystemColorMode.Dark) in Program.cs file
4. Run application, tab or mouse over this checkBox to observe the keyboard tooltip or mouse tooltip

[willibrandon]

Not sure why, but it seems the TTM_SETTIPBKCOLOR message is ignored when visual style is enabled on the control. If I disable it, then I can set the ToolTip BackColor and ForeColor.

Could a possible fix could be something similar to how the ToolTip is made contrasted enough in HC mode?

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

Lines 717 to 720 in c9aa1c9

	if (SystemInformation.HighContrast)
	{
	PInvoke.SetWindowTheme(HWND, string.Empty, string.Empty);
	}

[willibrandon]

The unit test would be to retrieve the background color and text color for a tooltip window using messages TTM_GETTIPBKCOLOR and TTM_GETTIPTEXTCOLOR, and assert the values returned are equal to SystemColors.Info and SystemColors.InfoText.

I believe that test fails right now when Dark mode is enabled.