The BackColor & MouseOverBackColor property of Button doesn't work

[Amy-Li03]

• .NET Core Version:
  .Net SDK 6.0.100-alpha.1.20421.6

• Have you experienced this same bug with .NET Framework?:
  No

• More info:
  This is a regression issue, it cannot reproduce on .Net SDK 5.0.100-preview.8.20417.9

Problem description:
The BackColor of Button doesn't work when Button's FlatStyle is Flat or Popup .
The MouseOverBackColor of Button doesn't work when Button's FlatStyle is Flat.

Expected behavior:
After run the project, the Button's BackColor should be YellowGreen;
When mouse over on the button, Button's BackColor should be Firebrick.

Minimal repro: DemoApp.zip

1. Create a new winforms .NET core project and target to .NET 5.0.
2. Add a Button to the form designer.
3. Add the following code to the structure method of Form1.
   button1.FlatStyle = FlatStyle.Flat;
   button1.BackColor = Color.YellowGreen;
   button1.FlatAppearance.MouseOverBackColor = Color.Firebrick;
4. Build and run the project.
5. Observe the BackColor of Button, then mouse over on the button and observe the BackColor of Button.

[RussKie]

The regression was introduced in 99be98a

[Jessie-Zhang01]

Verified this issue with .Net SDK 5.0.100-rc.2.20459.18, this issue is fixed. After running the project, the Button's BackColor is YellowGreen; When mouse over on the button, Button's BackColor is Firebrick.