Is it possible to set somewhere in configuration the color of the border between windows from red to black or remove the border completely?
Change the red border between windows in xmonad
xmonad
To change the colour, you can set normalBorderColor and focusedBorderColor in xmonad.hs. They should be set to #RRGGBB. For example, to set the focused window's border to black, set focusedBorderColor = #000000. The default color is #FF0000.To disable the border, set borderWidth = 0 in xmonad.hs, or use XMonad.Layout.NoBorders for specific layouts.