[Dark Mode] Button with FlatStyle 'Standard' is not in dark mode on 100% dpi after enabled SystemColorMode.Dark

### .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: https://github.com/dotnet/winforms/pull/11857

### Issue description

On 100% DPI, Button with FlatStyle 'Standard' is not in dark mode after enabled SystemColorMode.Dark, it cannot repro when monitor's dpi is greater than 100%.

on 100%DPI:
<img width="61" alt="image" src="https://github.com/user-attachments/assets/5f344eb3-c2ec-45ea-8c0f-21915133bdb2">

on 150%DPI:
<img width="87" alt="image" src="https://github.com/user-attachments/assets/132bba48-8210-42a9-88f6-fb9b80494859">


### Steps to reproduce

1. Create a WinForms .NET application with button control, set button's FlatStyle == Standard
2. Enable DarkMode by Application.SetColorMode(SystemColorMode.Dark) in Program.cs file
3. Run the application to observe

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[Dark Mode] Button with FlatStyle 'Standard' is not in dark mode on 100% dpi after enabled SystemColorMode.Dark #11949

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[Dark Mode] Button with FlatStyle 'Standard' is not in dark mode on 100% dpi after enabled SystemColorMode.Dark #11949

Description

.NET version

Did it work in .NET Framework?

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

Issue description

Steps to reproduce

Metadata

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

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