Dark mode support in WinForms backend

### What is the problem or limitation you are having?

With #4331 landed, we can now use .NET Core 10, which has full support for dark mode.

### Describe the solution you'd like

Work out the necessary API(s) to enable Dark Mode on .NET Core 10 applications, and audit all widgets' use of color to make sure they support dark mode properly. (Tree widgets currently always paint expanasion arrows in black and thus cannot be seen easily in dark mode).

### Describe alternatives you've considered

Status quo; light mode on WinForms is actually pretty.

### Additional context

The call seems to be Application.SetColorMode(SystemColorMode.System) at application startup.

https://learn.microsoft.com/en-us/dotnet/api/system.windows.forms.application.setcolormode?view=windowsdesktop-10.0

Start with the WinForms backend and the Application.SetColorMode(SystemColorMode.System) entry point documented in the issue. Audit each widget's color handling, paying particular attention to tree expansion arrows. Done means .NET Core 10 applications can enable dark mode and all widgets remain legible in dark mode.

Written by the indexing model from the issue text.