.NET version

.NET 10 with 10.0.100-rc.2.25468.104 build

Did it work in .NET Framework?

Not tested/verified

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

DarkMode is a new feature in .NET 10

Issue description

When using a Button with FlatStyle = Flat in a WinForms application, setting properties under the FlatAppearance property (such as BorderColor, BorderSize, MouseDownBackColor, MouseOverBackColor, etc.) does not work or apply correctly under app's Dark Mode
Like this setting code:

button2.FlatAppearance.BorderColor = Color.Fuchsia;
button2.FlatAppearance.BorderSize = 5;
button2.FlatAppearance.CheckedBackColor = Color.Yellow;
button2.FlatAppearance.MouseDownBackColor = Color.FromArgb(255, 255, 128);
button2.FlatAppearance.MouseOverBackColor = Color.Blue;
button2.FlatStyle = FlatStyle.Flat;

Dark Mode: has a problem: FlatAppearance properties (like BorderColor, BorderSize, MouseDownBackColor, MouseOverBackColor) are ignored. The button renders with the default system style, and custom appearance settings are not visible.

Classic Mode: works well

Steps to reproduce

1. Create a Winforms .NET 10 project
2. Add a button to the form
3. Set the button's FlatStyle to Flat, and set the properties BorderColor, BorderSize, MouseDownBackColor, MouseOverBackColor with custom values under FlatAppearance for button
4. Enable the DarkMode in program.cs
   Application.SetColorMode(SystemColorMode.Dark);
5. Build and run app to observe button