tested on openbsd 6.6

customize xenodm(1) login screen

enable xenodm(1):

# pkg_add spleen
# rcctl enable xenodm
#

edit /etc/X11/xenodm/Xresources:

xlogin.Login.echoPasswd:       true
xlogin.Login.fail:             fail
xlogin.Login.greeting:
xlogin.Login.namePrompt:       \040login\040
xlogin.Login.passwdPrompt:     passwd\040

xlogin.Login.height:           180
xlogin.Login.width:            500
xlogin.Login.y:                320
xlogin.Login.frameWidth:       0
xlogin.Login.innerFramesWidth: 0

xlogin.Login.background:       black
xlogin.Login.foreground:       #eeeeee
xlogin.Login.failColor:        white
xlogin.Login.inpColor:         black
xlogin.Login.promptColor:      #eeeeec

xlogin.Login.face:             spleen-24
xlogin.Login.failFace:         spleen-24
xlogin.Login.promptFace:       spleen-24

edit /etc/X11/xenodm/Xsetup_0:

#!/bin/sh
xsetroot -solid black

logout to check the login screen.

use yubikey for login and ssh.