Dtyjlui

How do I remove Ubuntu gnome-shell theme and restore the original one? I already tried sudo update-alternatives --config gdm3.css (gnome-shell.css). However, this only done like 75% work done. The login screen is now in default gray colors as I wanted but when I suspend my computer login screen is purple again. I also see purple corners when gnome-shell is loading.

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]