I’m very much a fan of black or at least plain backgrounds on my system; there’s much less clutter and distraction. When I upgraded my laptop to Ubuntu 20.04, I naturally configure my user session’s background to plain black. However, the login screen insisted on displaying some image rather than the plain black I wanted. Here’s how I fixed this.

The User Image

Each user account has a wallpaper image selected, even if the user has selected to use a plain color for their wallpaper when logged in. lightdm displays this image by default, based on which user is in the process of logging in, rather than honoring the user’s plain color selection for the wallpaper.

The Expected Fix

A quick Google search indicated that I should be able to edit /etc/lightdm/lightdm-gtk-greeter.conf and set background=#000000. However, this didn’t work; the user wallpaper image continued to be displayed.

The Actual Fix

Option user-background = false must be set so that the user’s wallpaper image isn’t displayed.

I ended up with the following /etc/lightdm/lightdm-gtk-greeter.conf:

background = #000000
icon-theme-name = Adwaita
font-name = Noto Sans 12
user-background = false
hide-user-image = true

Subsequently to tracking down the user-background option, I discovered that this option can be selected via the lightdm-gtk-greeter-settings GUI tool; it’s the “Use user wallpaper if available” option. Under XFCE at least, the tool is available via Applications Menu ⇨ Setting Manager ⇨ LightDM GTK+ Greeter settings.