Boring with the default purple login screen background in Ubuntu 18.04? Well, here’s how to make a change by hacking the css file.

Ubuntu 18.04 switches to Gnome 3 Desktop and GDM display manager. Although there are options to change desktop wallpaper and lock screen background (it’s actually screen saver with date & time displayed), the login screen is always purple.

In the picture above, I’ve changed the login background to the image “Aardvark_Wallpaper_Grey_4096x2304.png” locate in /usr/share/backgrounds.

1. Move your favorite image to /usr/share/backgrounds:

It’s better to move the image you want to set as login background to system pictures folder: /usr/share/backgrounds.

To do so, open terminal via Ctrl+Alt+T, then either run command:

sudo cp PATH/TO/YOUR/IMAGE /usr/share/backgrounds/

or run command to open file browser via root and then do copy and paste in that window:

xhost +local: && sudo nautilus /usr/share/backgrounds/

2. Edit the css file that define GDM login background:

Open terminal (Ctrl+Alt+T) and run command to edit file /etc/alternatives/gdm3.css, which is linked to /usr/share/gnome-shell/theme/gdm3.css:

xhost +local: && sudo gedit /etc/alternatives/gdm3.css

Input your password (no visual feedback while typing) when it prompts and hit Enter. And of course back up the file before editing.

3. When the file opens, go to menu (right after ‘Save’ button) -> find and find out following section:

#lockDialogGroup {
  background: #2c001e url(resource:///org/gnome/shell/theme/noise-texture.png);
  background-repeat: repeat; }

change the lines into:

#lockDialogGroup {
  background: #2c001e url(file:///usr/share/backgrounds/Aardvark_Wallpaper_Grey_4096x2304.png);
  background-repeat: no-repeat;
  background-size: cover;
  background-position: center; }

Replace Aardvark_Wallpaper_Grey_4096x2304.png with your picture file name.

Finally save the file and restart your computer to apply changes.

NOTE: these changes may be overwritten in case of an system update with GDM packages. Please let me know if you find a good way to change the GDM3 login background in Ubuntu 18.04.