I currently have the Colloid theme installed, but this occurs with every theme I try to install. First, I suspected the blur-my-shell extension, but even after disabling all my extensions, it still persists. I'm using Zorin OS 18
I'm sorry but if you don't write it correctly or forget spaces and ';' I can't help, if it's work for all of us it should work for you, CSS is code and this is not only text you put in a place you want, respect structure and read the topic I gave you to read, till the end
and when you change something in this code you have to restart your gnome session.
ALT + F2 > r