What makes Linux appealing to me is the extent of customizability, but I didn’t find many answers when looking up with desktop environment is them most customizable. Some say KDE is most customizable than say, Gnome, but doesn’t Gnome support CSS customization while KDE doesn’t?
CSS is just another tool in a toolbox. Having it doesn’t make one more customizable than the other, being more customizable does. Customization means options. Things you can set, things you can change. Not HOW they’re changed.