It's a work in progress.
Many frequently used windows got dark mode support in 6.6:
https://winscp.net/eng/docs/history?a=6.6
Issue 1696 – Dark theme
Others might follow in future versions.

And "Rename tab"

"Menu" "Synchoronize" and "Edit Link"

Hello Martin,

I'm currently testing WinSCP 6.6 and noticed that both the “About” and “Preferences” "Filter" and "Menu" "Synchoronize" and "Edit Link" and "Rename tab" dialogs still display in light mode, even when the main interface is using dark mode.

It would be great if these secondary dialogs could also follow the dark theme.
This would improve visual consistency and make the experience more comfortable for users who prefer or always work in dark environments.

Thanks for considering this enhancement — the dark mode implementation in WinSCP is already looking great overall!