ubuntu lock screen not changing background

Before you mark this as duplicate, I've tried everything out there. I've been fiddling with gnome tweaks and added a theme by following this tutorial, https://www.omgubuntu.co.uk/2017/03/make-ubuntu-look-like-mac-5-steps I tried changing the, I followed this tutorial to change the background, https://vitux.com/how-to-change-login-lock-screen-background-in-ubuntu/. In a gist, it says to change this file /usr/share/gnome-shell/theme/ubuntu.css which I did. When I lock my screen by pressing ⊞ Win + L it shows up but when I go restart my pc it never shows up. My theme is stored in a .themes folder in my home folder. I use the basic gdm3 which can installed when installing ubuntu 18.04. It is quite annoying that it doesn't persist.