It's similar issue as here:
Issue 2080 – More contrasting or configurable file panel selection color (particularly for the dark mode)
I'm still hoping that Microsoft improves dark mode API for apps. But it does not look like they would (it's been years).
The only other solution is that WinSCP draws the panels on its own, instead of relying on Windows API. But that's something I wanted to avoid.

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In Dark Mode, is there a way to make visible the file Tree arrow symbols that show whether a folder is expanded or not expanded? They show if you hover over them, but otherwise they display dark on dark and can't be seen.

I'm using WinSCP version 5.21.1 Build 12643 on Windows 10 build 19044. I usually use the Explorer interface but they don't show in Commander interface either. The arrow symbols have never been visible for me in Dark Mode ever since Dark Mode was introduced.