We can change the login appearance via the menu Preferences – LightDM Appearance.
This will give us a pop up of the following screens. Most of these elements are straight forward.
Changing the icons to a personal picture or avatar is a bit more complex and is explained in this article.
You can change theme, icon theme, font, background image.
Those will be the most common settings, that you want to change but check out the other settings in the images.