Apply theme on login screen

for Arc-Theme, in Ubuntu 18.04.

Ubuntu 18.04 change Login Screen theme (not wallpaper) following this Answer and Warnings in it.

I have tried to change login & lock screen themes same as User Shell Theme Arc-Theme

when you download the arc-theme from https://github.com/horst3180/arc-theme github. simplfy it like below

this Arc-Theme is lacking content for some keys for Onscreen-Keyboard.

so, i have adapted that from default file /usr/share/gnome-shell/theme/ubuntu.css

copy the lines 1549 to 1566 from /usr/share/gnome-shell/theme/ubuntu.css and paste under line number 1708 in /usr/share/themes/Arc-Theme/gnome-shell/gnome-shell.css

  .keyboard-key.default-key {
    border-color: #32322d;
    background-color: #20201d;
    background-size: 20px; }
  .keyboard-key.enter-key {
    border-color: #dd4814;
    background-color: #e95420;
    background-image: url("resource:///org/gnome/shell/theme/key-enter.svg"); }
  .keyboard-key.shift-key-lowercase {
    background-image: url("resource:///org/gnome/shell/theme/key-shift.svg"); }
  .keyboard-key.shift-key-uppercase {
    background-image: url("key-shift-uppercase.svg"); }
  .keyboard-key.shift-key-uppercase:latched {
    background-image: url("key-shift-latched-uppercase.svg"); }
  .keyboard-key.hide-key {
    background-image: url("resource:///org/gnome/shell/theme/key-hide.svg"); }
  .keyboard-key.layout-key {
    background-image: url("resource:///org/gnome/shell/theme/key-layout.svg"); }

save the file & close.

now run the below command:

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

now copy the 3 folders common-assets, dark-assets & light-assets from /usr/share/themes/Arc-Theme/gnome-shell/ to /usr/share/gnome-shell/theme/

now copy paste all the content from /usr/share/themes/Arc-Theme/gnome-shell/gnome-shell.css to /usr/share/gnome-shell/theme/ubuntu.css file replacing the data in ubuntu.css file.

save the file, close & shutdown

fresh start the system to see the change.

Lock Screen animation:

Login Screen Pictures:

Actually just figure out the proper way to do this today by adding options to update-alternatives.

First, you can view the current setting of gdm3.css using

update-alternatives --display gdm3.css

You should see the following text.

gdm3.css - manual mode
  link best version is /usr/share/gnome-shell/theme/Yaru/gnome-shell.css
  link currently points to /usr/share/gnome-shell/theme/Yaru/gnome-shell.css
  link gdm3.css is /usr/share/gnome-shell/theme/gdm3.css
/usr/share/gnome-shell/theme/Yaru/gnome-shell.css - priority 15
/usr/share/gnome-shell/theme/gnome-shell.css - priority 10

I have installed the Adapta themes, and I would like to add Adapta-Eta theme to the options for gdm3.css. The path of the gdm3.css for Adapta-Eta theme is /usr/share/themes/Adapta-Eta/gnome-shell/gdm3-Adapta-Eta.css.

To install the Adapta-Eta gdm3.css,

sudo update-alternatives --install /usr/share/gnome-shell/theme/gdm3.css gdm3.css /usr/share/themes/Adapta-Eta/gnome-shell/gdm3-Adapta-Eta.css 20

update-alternatives --install link name path priority

you should use the link gdm3.css is ... from the text you get using the --display option as link.

name should be gdm3.css

path is the path to the new option

you can set any integer for priority (I don't know what difference it makes)

Now to change the theme of the login screen, use sudo update-alternatives --config gdm3.css

Choose the new theme you want to use. You should be able to see the change after reboot.

I'm not sure if the upgrade of the system will modify it, but at least when the theme is updated it would be overridden.

Although this is one issue, the lock screen appears to not changing according to the choice. Only the login screen follows the choice.