bspwm

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.