[Dark Mode] TextBox and MaskTextedBox Control backcolor is not dark When set the readonly property is true

[MelonWang1]

.NET version

.NET SDK 9.0.100-rc.1.24415.1 + build private dll form winforms repo release/9.0 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

TextBox and MaskTextedBox Control backcolor is not dark when set the readonly property is true.

Steps to reproduce

1. Create a WinForms .NET Core application
2. Enable DarkMode by Application.SetColorMode(SystemColorMode.Dark) in program.cs file.
3. Add a TextBox and MaskTextedBox Control and set their readonly property to true.
4. Run the application.
5. Observe the result.

[KlausLoeffelmann]

That's a tricky one, which could be somewhat easily addressed.
For now the workaround would be to test, if the control is in Darkmode and in ReadOnly, and then set the color accordingly.
This needs discussion, which colors would be appropriate to use here, and we should schedule this for .NET 10.