Gfilui

I'm running 18.04 LTS, and I've changed both my desktop wallpaper and the login screen wallpaper to a custom image. However, when I boot my computer, The login screen will load and display the default purple with grid (from Unity? I had Unity installed alongside GNOME but I removed it) before loading my image. Then when I log in to my account, my image is replaced with that default purple fuzzy looking wallpaper for a a few seconds before loading my image again as the desktop wallpaper. Is there any way to change this? Its annoying to have these purple themes load between my image as I would like it to be a smooth boot up and log in. I'm fairly new at Linux, so any help would be appreciated!

Edit: I changed the login screen background by editing the gdm3.css file and using the following in the terminal:

sudo -i 
xhost +SI:localuser:lightdm 
su lightdm -s /bin/bash 
gsettings set com.canonical.unity-greeter draw-user-backgrounds 'true' 
gsettings set com.canonical.unity-greeter background 'path-to-image' 

Also, to clarify, I installed the Unity desktop environment alongside the default GNOME in 18.04, but have since deleted it. However, I still get the Unity style log in screen with the background grid and the password entry box to the right rather than the GNOME one with the password entry box in the center of the screen.

My custom image wallpaper does display on the login screen, I just get the defaults before it loads. So after grub, I get the default Unity login wallpaper, then my login wallpaper, and then when I log in I get the default GNOME login wallpaper, then my custom image desktop wallpaper.

UPDATE: After running the command

sudo dpkg-reconfigure lightdm 

and changing the display manager to GDM rather than lightdm, the default screen flash of the unity login screen has gone away, however, I still get the fuzzy purple GNOME login screen flash (after typing in my password) between my wallpapers when logging in.

Changing gdm3.css: Both /etc/alternatives/gdm3.css and /usr/share/gnome-shell/theme/gdm3.css were changed to:

#lockDialogGroup 
 background: #2c001e url(file:///usr/share/backgrounds/5a4f053d3cac434ada0c1cfda611629a.jpg);
 background-repeat: no-repeat;
background-size: cover;
background-position: center; 

The oringinal was:

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