There is no file, however there is a way to change the color.
Run the following commands:
echo "[com.canonical.unity-greeter]" | sudo tee /usr/share/glib-2.0/schemas/10_unity_greeter_background.gschema.override
echo "draw-user-backgrounds=true" | sudo tee -a /usr/share/glib-2.0/schemas/10_unity_greeter_background.gschema.override
echo "background-color='#000000'" | sudo tee -a /usr/share/glib-2.0/schemas/10_unity_greeter_background.gschema.override
sudo glib-compile-schemas /usr/share/glib-2.0/schemas/
Of course, this changes the screen to black.
More setting options such as setting the background to a static image can be found at the source link below.
source
to change the login screen background in Ubuntu 18.10 with default Display Manager that is gdm3
we need to edit the gdm3.css file.
open the file with your favorite editor, mine is gedit. so
sudo -H gedit /etc/alternatives/gdm3.css
go to line number around 1981 and find the below content.
#lockDialogGroup {
background: none;
background-color: none;
background-gradient-direction: vertical;
background-gradient-start: #6D2169;
background-gradient-end: #370026; }
change the content with
#lockDialogGroup {
background: #000000 url(file:///home/pratap/Desktop/1.jpg);
background-repeat: no-repeat;
background-size: cover;
background-position: center; }
here in the line number 1982 url(file://put your actual image path)
save the file, close & reboot to see the change.
Picture of Lock Screen Wallpaper
Picture of Unlock Screen Background
for 18.04 same method, only line number varies 1814,
#lockDialogGroup {
background: #2c001e url(resource:///org/gnome/shell/theme/noise-texture.png);
background-repeat: repeat; }
replace the above content with
#lockDialogGroup {
background: #000000 url(file:///home/pratap/Desktop/1.jpg);
background-repeat: no-repeat;
background-size: cover;
background-position: center; }
here in the line number 1815 url(file://put your actual image path)
Best Answer
to change the login screen color equivalent to those 15 colors which are available when we open Settings>Background>LockScreen>color
first set the required color as lockscreen from the settings>background>lockscreen>color
to know the value of this color run the below command.
Example:
Copy the color, in above case it is
#7a8aa2
now we need to edit the gdm3.css file.
open the file with your favorite editor, mine is gedit. so
sudo -H gedit /etc/alternatives/gdm3.css
go to line number around 1985 & 1986 and find the below content.
change the content with
here in the line number 1985 & 1986 put the color you want in place of #7a8aa2
save the file, close & reboot to see the change.
these are the 15 colors + default one.