Ubuntu gnome-shell purple design

if i understood your question correctly, you want to go back to default theme.. you can use the same command again and select the number 0

sudo update-alternatives --config gdm3.css

$ sudo update-alternatives --config gdm3.css
[sudo] password for mkr: 
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/gnome-shell.css        10        manual mode

Press <enter> to keep the current choice[*], or type selection number: 0
update-alternatives: using /usr/share/gnome-shell/theme/Yaru/gnome-shell.css to provide /usr/share/gnome-shell/theme/gdm3.css (gdm3.css) in auto mode
$ 

If you want to apply the changes to unlock-screen also, this answer may help.
https://askubuntu.com/a/1107377/739431

Edit: as per the comments, you have manually deleted the folder Yaru to install yaru-theme package run the below command,

sudo apt install yaru-theme-gnome-shell

$ sudo apt install yaru-theme-gnome-shell
[sudo] password for mkr: 
Reading package lists... Done
Building dependency tree       
Reading state information... Done
yaru-theme-gnome-shell is already the newest version (18.10.6).
yaru-theme-gnome-shell set to manually installed.
0 upgraded, 0 newly installed, 0 to remove and 17 not upgraded.
$ 

It says this is already the newest version, may be because we have manually deleted the folder Yaru

so, sudo apt purge yaru-theme-gnome-shell

$ sudo apt purge yaru-theme-gnome-shell
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following package was automatically installed and is no longer required:
  xwayland
Use 'sudo apt autoremove' to remove it.
The following packages will be REMOVED:
  ubuntu-desktop* ubuntu-session* yaru-theme-gnome-shell*
0 upgraded, 0 newly installed, 3 to remove and 17 not upgraded.
After this operation, 637 kB disk space will be freed.
Do you want to continue? [Y/n] 

Please note that the command sudo apt purge yaru-theme-gnome-shell will also remove ubuntu-desktop* and ubuntu-session* along with yaru-theme-gnome-shell*

Once we continue with Yes,

we need to again install ubuntu-desktop and ubuntu-session packages..

sudo apt install ubuntu-desktop ubuntu-session

$ sudo apt install ubuntu-desktop ubuntu-session
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following additional packages will be installed:
  yaru-theme-gnome-shell
The following NEW packages will be installed:
  ubuntu-desktop ubuntu-session yaru-theme-gnome-shell
0 upgraded, 3 newly installed, 0 to remove and 17 not upgraded.
Need to get 113 kB of archives.
After this operation, 637 kB of additional disk space will be used.
Do you want to continue? [Y/n] 

You can hack at the CSS directly too.

I wrote a how-to on removing purple from the Ubuntu theme.

http://teknorants.blogspot.com/2020/01/changing-ubuntus-royale-purple.html

Just install gnome-session. Then log into it (for example, "GNOME on Xorg").

Then enable Ubuntu extensions:

gnome-shell-extension-tool -e [email protected]
gnome-shell-extension-tool -e [email protected]

And change wallpaper for lock screen.