gdm - login screen background change problem - Ubuntu 18.04 LTS

Question

I am trying to change the noisetexture.png to black screen but it still flashes a purple screen while logging in .

I changed this

#lockDialogGroup {
   background: #2c001e url(resource:///org/gnome/shell/theme/noise-texture.png);
   background-repeat: repeat; 
}

to

#lockDialogGroup {
   background: #00000 url(file:///home/sk/image.png);
   background-repeat: no-repeat; 
   background-size: cover;
   background-position: center;}

How to solve this issue?