Customize separator

How can I change the width and color of the buffer separators? I found that the vertical one is called right-divider, but I don’t know how to change it.

Doom uses the window-divider package (built into Emacs) to control them.

To change their size:

;; In $DOOMDIR/config.el
(setq window-divider-default-right-width 4
      window-divider-default-bottom-width 4)
;; Doom's default for these is 1

To change their color, adjust the vertical-border face (it’s actually window-divider, but most themes – include Doom’s – have it inherit from vertical-border). E.g.

;; in $DOOMDIR/config.el
(custom-set-faces! '(vertical-border :foreground "red"))