A minimal dark configuration for the XLogin prompt.  Back up
your /etc/X11/xdm/Xresources and replace it with the following.
This example uses spectrum's <https://github.com/cmvnd/fonts> bitmap
font, but you can replace it with whatever compels you most. You may
also add a somthing lile `xsetroot -solid #151515` to get a consistent
background color.

xlogin.Login.greeting:            X Display Manager
xlogin.Login.unsecureGreeting:
xlogin.Login.fail:                wrong guess.
xlogin.Login.changePasswdMessage: change.
xlogin.Login.namePrompt:          username:
xlogin.Login.passwdPrompt:        password:
xlogin.Login.echoPasswd:          true
xlogin.Login.background:          #1c1b19
xlogin.Login.foreground:          #fce8c3
xlogin.Login.failColor:           #e02c6d
xlogin.Login.inpColor:            #1c1b19
xlogin.Login.promptColor:         #baa67f
xlogin.Login.greetColor:          #baa67f
xlogin.Login.face:                antidote:size=8
xlogin.Login.failFace:            antidote:size=8
xlogin.Login.promptFace:          antidote:size=8
xlogin.Login.greetFace:           antidote:size=8
xlogin.Login.frameWidth:          0
xlogin.Login.innerFramesWidth:    0
xlogin.Login.sepWidth:            0
xlogin.Login.width:               240
xlogin.Login.height:              135