Form's FormBorderColor property value always is Black although has changed it to another color value

[John-Qiao]

.NET version

.NET SDK 9.0.100-rc.1.24415.1 + build private dlls from 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+?

No, this is a new feature Dark Mode: #11857

Issue description

Form's FormBorderColor property value always is Black although has changed it to another color value.

Actual result:
The value is 'Black' in textbox control.

Expected result:
The value should be 'Green' in textbox control.

More info:
This issue occurs in FormCaptionBackColor and FormCaptionTextColor properties too.

Steps to reproduce

1. Launch the FormBorderColor-TestApp.zip project in VS and run it.
2. Observe the value in textbox control.

[John-Qiao]

Verified this issue on 9.0.100-rc.2.24419.18 with dlls built from winforms repo of main branch, it was fixed: the values all are correct in textbox controls now.

[MandiMan]

@KlausLoeffelmann verified this issue in the latest .NET 9 RC1 build: 9.0.100-rc.1.24421.9 from https://aka.ms/dotnet/9.0.1xx-rc1/daily/dotnet-sdk-win-x64.exe, which partial fixed that can get the expected values "Green" in textbox controls, but FormBorderColor, FormCaptionBackColor, FormCaptionTextColor are not set to green.

Write the code in form_load, it was fixed, but in constructor, partial fixed.

[MandiMan]

Verified the issue with .NET 9.0.100-rc.1.24422.10 test pass build that the issue partial fixed in constructor and form_load. We can get the expected values "Green" in textbox controls, but FormBorderColor, FormCaptionBackColor, FormCaptionTextColor are not set to green.

[Nora-Zhou01]

Verified this issue in the latest .NET9.0 SDK build: 9.0.100-rc.2.24428.9, write the code in form_load, it was fixed, but in constructor, partial fixed.