Total Commander

1. It doesn't fit anywhere in the configuration

I could just be an item of its own, instedad of a subitem.

It works fine here in 32bit TC with and with XP-themed background:
https://tcmd.madsenworld.dk/nohorizontallines32.png
https://tcmd.madsenworld.dk/nohorizontallines32themed.png
https://tcmd.madsenworld.dk/nohorizontallines32Win10.png
https://tcmd.madsenworld.dk/nohorizontallines64.png

Of course I have to use another color for the lines when I enable XP-themed background

great, thank you. it works ok in dark mode

Currently it doesn't work properly in the 32-bit version: The line below the menu gets overdrawn all the time by the Windows theme.

You cannot turn the Windows theme for the Menu off in the 32bit version anyway - https://ghisler.ch/board/viewtopic.php?t=79290

It still works fine for removing the lines in the buttonbar, which is nice, especially when there are more than one row of buttons.

So I think making i applicable via the Colors configuration, for both Normal and Dark mode is OK.