@Bzv021Whatever I have done it is always white and in that case I have issue with a font,
which is also white for dark mode.
As far as i can tell this is not possible,
because i'm seeing no Option inside the palette.txt File for that as well.
In that case you have to match the Font Color for the white Background within the Drop Down Menu.
Like
@stefnev1 already stated you can change the Fontcolor for the Buttons and
the Buttoncolour itself, for that you have these two Values.
Button = #ffffff ; Background of regular buttons.
ButtonText = #000000 ; Foreground of regular buttons.
But it has no Effect for the Background inside the Drop Down Menu.