I would like to replace green with other colors. Also, I don’t want to install other software to make it just use nano to change CSS liked file. Does anyone have ideas?
When I was a user of Gnome desktop I found it incredibly difficult to accomplish. You will have to unpack or decompile the resource file change, the css and then recompile - and watch out not doing anything unsupported or gdm refuses to load.
You will probably have more success replacing the gdm display manager with e.g. lightdm, lxdm or sddm.