The people who are wanting to use this in Linux Mint Cinnnamon 20. first you must install lightdm-gtk-greeter and its settings by this command

sudo apt install lightdm-gtk-greeter lightdm-gtk-greeter-settings

Next, you have to download the theme and extract it in the Downloads folder. then run this command to the theme to the /usr/share/themes folder

sudo mv ~/Downloads/ /usr/share/themes

Next, you have to open LightDM GTK+ Greeter settings by searching it in the menu and open it with your password. Then you can choose the gtk+ theme from there and save your configuration.

Next we have to create a configuration to tell lightdm to use the greeter that we are going to specify. To do that I will be using nano as my editor.
Run the command:

sudo nano /etc/lightdm/lightdm.config

then paste the following line

[SeatDefaults]
greeter-session=lightdm-gtk-greeter

then save the file and exit nano. After that you can check of lighdm is using our specified greeter by running this command:

lightdm --show-config

If you see greeter-session=lightdm-gtk-greeter, the you are all fine.

Then run the following command to restart lighdm so as to see the login screen:

sudo service lightdm restart

Then you should get the login screen with you theme.

How do I install this?

I think the file you need to edit is /etc/lightdm/lightdm.conf.

extract to /usr/share/themes/. must be root or sudo. There is an app to configure lightDM. https://packages.debian.org/search?keywords=lightdm-gtk-greeter-settings

There is a way to configure manually but it has been so long I don't remember. https://wiki.debian.org/LightDM