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.
gsettings get org.gnome.desktop.screensaver primary-color
Example:
pratap@i7-6550U:~$ gsettings get org.gnome.desktop.screensaver primary-color
'#7a8aa2'
pratap@i7-6550U:~$
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.
#lockDialogGroup {
background: none;
background-color: none;
background-gradient-direction: vertical;
background-gradient-start: #6D2169;
background-gradient-end: #370026; }
change the content with
#lockDialogGroup {
background: none;
background-color: none;
background-gradient-direction: vertical;
background-gradient-start: #7a8aa2;
background-gradient-end: #7a8aa2; }
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.
#7a634b
#425265
#475b52
#888888
#7a8aa2
#ff89a9
#ed9116
#d272c4
#437740
#fad166
#ab2876
#5d479d
#008094
#db5d33
#000000
#023c88
sudo gedityou should probably change that tosudo -H gedit, so as you don't accidentally create a login loop. Usesudo -Hwhenever you start a graphic app from the command line. Cheers! – heynnema Nov 10 '18 at 17:19#lockDialogGroupis different in/etc/alternatives/gdm3.css, and modifying the background color there doesn't work. If you'd like me to start a new question for this, please let me know :-) – heynnema Jun 12 '19 at 15:01#lockDialogGroup { background-color: #2C001E; }. – heynnema Jun 12 '19 at 15:20#lockDialogGroup { background-color: #2C001E; }to#lockDialogGroup { background-color: #7A8AA2;} ##2C001E; }. Maybe I forgot the trailing;I wonder if that's it, or if it didn't like that I made the last line a comment (so I knew what to go back to if I needed to...). – heynnema Jun 12 '19 at 15:38