bspwm - adigitoleo/vim-mellow GitHub Wiki
bspwm is a stadalone window manager for X sessions
grep color ~/.config/bspwm/bspwmrc
bspc config normal_border_color '#66292F' # unfocused window
bspc config active_border_color '#66292F' # focused, on unfocused monitor
bspc config focused_border_color '#FF7477' # focused, on focused monitor
Or adapt the config commands for your tiling WM of choice.