18.04 - Cannot change the background color of the LOCK screen, not the Login screen

Question

Good evening,

I'm trying to change some of the colours of my login screen in my Ubuntu 18.04. I followed some questions here on AskUbuntu, and edited my css files at /usr/share/gnome-shell/theme/gnome-shell.css (I'm using that configuration) - Specifically, I changed the values of .lockDialogGroup to a nicer dark gray.

My login screen is indeed now a nice dark gray instead of that hellish purple.

However, if I lock my computer without shutting down the machine, I still get a different purple gradient, which I haven't managed to change. I'm serching around but since I don't even know what to call this "behind-the-shield" screen, I can't find relevant results.

I suspect the login screen and the lock screen are two different managers altogether.

Here's a short mp4 video of me literally filming my screen, to make you understand which color I'm trying to change:

https://i.sstatic.net/VdEPP.jpg

EDIT: There seems to be a discrepancy between my first-after-boot login screen, and my login-after-locking screen.

first Login screen after boot:

After-lock login screen:

Answer 1

Edit the file /usr/share/xsessions/ubuntu-communitheme-snap.desktop and change the "ubuntu-communitheme" text to "myct" (change anything, not necessarily "myct") in the line starting with Exec=

sudo -H gedit /usr/share/xsessions/ubuntu-communitheme-snap.desktop

[Desktop Entry]
Name=Ubuntu with communitheme snap
Comment=This session logs you into Ubuntu
Exec=env GNOME_SHELL_SESSION_MODE=myct /snap/communitheme/current/session
TryExec=/snap/communitheme/current/session
Type=Application
DesktopNames=communitheme:ubuntu:GNOME
X-Ubuntu-Gettext-Domain=gnome-session-3.0

create a file named myct.json in the directory /usr/share/gnome-shell/modes/ with below content.

{
    "parentMode": "user",
    "stylesheetName": "myct.css",
    "enabledExtensions": ["ubuntu-dock@ubuntu.com", "ubuntu-appindicators@ubuntu.com"]
}

Create a text file named myct.css in the directory /usr/share/gnome-shell/theme/

sudo touch /usr/share/gnome-shell/theme/myct.css

Open the file and paste the below content

sudo -H gedit /usr/share/gnome-shell/theme/myct.css

Content:

@import url("/snap/communitheme/current/share/gnome-shell/theme/Communitheme/gnome-shell.css");

#lockDialogGroup {
  background-color: magenta; }

Reboot and then lock the screen to see your preferred color.