Change Ubuntu Login Purple Background

Using the terminal, edit ubuntu.css

sudo gedit /usr/share/gnome-shell/theme/ubuntu.css

Update the following CSS element

#lockDialogGroup {
  background: #2c001e url(file:////usr/share/backgrounds/black-background.jpg);
  background-repeat: no-repeat; 
  background-size: cover;
  background-position: center;
}