Ubuntu 18.04 change Login Screen theme (not wallpaper)

I can't seem to find any way to change the default orange on black theme on Ubuntu 18.04. I am using Adapta theme on my desktop, but it does not get applied to the login screen. Please notice that I am not talking about the purple background, I am talking about the theme itself.

Solution 1:

Disclaimer: playing with gdm3 files is very dangerous, small mistakes will stop access to gnome desktop session. Only tty will work. i have several times reinstalled Ubuntu in such cases. although i have successfully applied User Shell Theme to Login Screen & Lock Screen. Do this on your own risk.

I am going to apply User Shell Theme Adapta-Nokto to Login Screen and Lock Screen

for Ubuntu 18.04 only

files or folders to know:
1./etc/alternatives/gdm3.css
2./usr/share/gnome-shell/theme/ubuntu.css
3./usr/share/themes/Adapta-Nokto

take backups (mandatory):
1. /usr/share/gnome-shell/theme/ubuntu.css file. (say ubuntu.css backup)
2. /usr/share/gnome-shell/theme folder. (say theme backup)

https://we.tl/t-iDLEPaXoI0 5MB .gif file showing default lock screen theme.

Concept:
nothing below i wrote is official, based on several trail and errors i found this. proceed with your own risk.

*after several trails i found that /etc/alternatives/gdm3.css is playing a very big role. this file is controlling the log in screen beauty and not lock screen.

*/usr/share/gnome-shell/theme/ubuntu.css is controlling the lock screen beauty.

by default, the content in /etc/alternatives/gdm3.css file and /usr/share/gnome-shell/theme/ubuntu.css file are same because /etc/alternatives/gdm3.css file is a linked file to /usr/share/gnome-shell/theme/ubuntu.css

we need to break this link. and create a link to our User Shell Theme.

to break the link and tell log in screen to use User Shell theme,
run the below command:

sudo rm /etc/alternatives/gdm3.css && sudo ln -s /usr/share/themes/Adapta-Nokto/gnome-shell/gnome-shell.css /etc/alternatives/gdm3.css

Example:

pratap@i7-4770:~$ sudo rm /etc/alternatives/gdm3.css && sudo ln -s /usr/share/themes/Adapta-Nokto/gnome-shell/gnome-shell.css /etc/alternatives/gdm3.css
[sudo] password for pratap: 
pratap@i7-4770:~$

we need to copy the assets folder and some more image files from /usr/share/themes/Adapta-Nokto/gnome-shell/ folder to /usr/share/gnome-shell/theme/

now log in screen will use this theme. to test it, shutdown the system. fresh start.

https://we.tl/t-uVLyw8mVga 5MB .mp4 file showing log in screen changed.

https://we.tl/t-jFZINc18q7 5MB .gif file showing lock screen unchanged.

note that, lock screen will not use this theme. as of now we changed only log in screen theme.

to change the lock screen theme, copy paste all the content from /usr/share/themes/Adapta-Nokto/gnome-shell/gnome-shell.css to /usr/share/gnome-shell/theme/ubuntu.css file replacing the data in ubuntu.css file.

reboot to see the change.

https://we.tl/t-8oi9JRTRpM 5MB .gif file showing lock screen changed.

want to go back to default?

copy paste all the content from backed up (say ubuntu.css backup) file into /usr/share/gnome-shell/theme/ubuntu.css file replacing the data in ubuntu.css file.

run the below command:

sudo rm /etc/alternatives/gdm3.css && sudo ln -s /usr/share/gnome-shell/theme/ubuntu.css /etc/alternatives/gdm3.css

copy the images from backed up folder (say theme backup) and make the folder look like this

Reboot to see default look.

Solution 2:

There is an easy way to Install/Switch/Remove GDM3 theme using update-alternatives:

update-alternatives --install <link> <name> <path> <priority> 
add a group of alternatives to the system.

update-alternatives --config <name>
show alternatives for the <name> group and ask the user to select which one to use.

update-alternatives --remove <name> <path> 
remove <path> from the <name> group alternative.

In this example we will change default(Ubuntu 18.04) GDM theme to Yaru:

Install new alternative for GDM3 theme(gdm3.css):

sudo update-alternatives --install /usr/share/gnome-shell/theme/gdm3.css gdm3.css /usr/share/gnome-shell/Yaru/gnome-shell.css 15

Alternative installed, now we can select wich one to use:

sudo update-alternatives --config gdm3.css


There are 2 choices for the alternative gdm3.css (providing /usr/share/gnome-shell/theme/gdm3.css).

  Selection    Path                                               Priority   Status
------------------------------------------------------------
* 0            /usr/share/gnome-shell/theme/Yaru/gnome-shell.css   15        auto mode
  1            /usr/share/gnome-shell/theme/Yaru/gnome-shell.css   15        manual mode
  2            /usr/share/gnome-shell/theme/ubuntu.css             10        manual mode

Press <enter> to keep the current choice[*], or type selection number: